Three-abreast in a 10'-wide passage plus reach weapons poking through from the second rank was and still is SOP round here if-when a party happens to have enough warriors to make it work. Far more often it's seen as an opposition tactic when the foes are halfway organized and don't have many (if any) backliners.
It was 3e that forced everyone into 5' squares. Before that, 3 feet would do and even less if the character was small e.g. a Gnome or Hobbit (or Goblin or Kobold, as many a party has found!) of which you could get 4 across a 10' passage.
1st didn’t really use grid based combat, but 3 ft. is probably derived from real world troops in ranks. But PCs more typically fight as individual skirmishers, so 5 ft is fair enough. It’s the difference between arms length and shoulder to shoulder.
although, as per 5e rules, a creature can squeeze into a space for a creature one size smaller. So goblins could stand 4 abreast in a 10 ft. space (with disadvantage).
1st doesn't really spell out using grids for combat, but uses them for exploration, so formations naturally arise from marching order in the latter. Remember also that in 1E that how many combatants you can squeeze into a given space isn't a fixed number like in later D&D with 5' squares or GURPS with its 3' hexes, but is also dependent on the weapon's space required to wield (specified on the weapon charts in the PH). A two handed sword with its 6' minimum space required to wield would normally mean only one fighter with a two-handed in a 10' corridor. Whereas longsword only requires 3', and a spear or short sword only 1'. (see PH page 38). The DMG tells us that three figures abreast in a 10' corridor is typical, but subject to adjustment based on circumstances (such as weapon space required).
That, and honestly, OD&D spears were inferior weapons which RQ ones (especially longspears) really weren't. Combine that with the fact a larger number of characters could use one effectively (pretty much all tribal types got trained in them) you just could easily end up being a simultaneously more practical and effective choice.
Sure. Though OD&D spears certainly aren't bad. If you're playing under 1974 rules all weapons do the same d6 damage anyway. Even once Longswords get upgraded to d8 in Greyhawk a year later (and better vs Large) you're still looking at getting twice as many attacks in the same frontage if you have a second rank with spears. Three attackers dealing d8s is strictly worse than three attackers dealing d8s AND three more dealing d6s. Or two and two if you're using a simpler 5' square per combatant rule (as was suggested by Moldvay in 1981 Basic and as became standard in the WotC editions).
I mean, let's say you were wandering around with a six character party and a couple of henchmen. The latter were going to primarily be human mules, because among OD&D groups it wasn't even often assumed they'd level, nor be treated as FM for traits, so putting them on the line wasn't attractive if you wanted to keep them around for their hauling duties. By a bit in, that'd probably be two Fighters, two MU, a Cleric and a Thief. With either 2 or 3 abreast lines, who was going to be doing that second row of spears?
I think part of why you and Lanehan were talking past each other a bit is a terminology clash.
AD&D uses the term Henchman to refer to classed and leveled elite auxiliaries, who have superior morale, capabilities, and autonomy compared to zero level Hirelings.
OD&D is a little more vague, but Book III divides hired "Men at Arms" into Fighters of various grades and Non-Fighters, and Book I specifies that you can hire characters of any class and race with appropriate incentives, but that "only the lowest level of character types can be hired."
1981 Basic doesn't really talk about zero-level hirelings but instead defaults to assuming that all Retainers are classed adventurers, of any level (though never higher level than the PC hiring them), and describes them as "more than just men at arms", but as "lieutenants and assistants to a PC [who] are expected to lend their skills and knowledge to the benefit of the party and take the same risks the characters expect to face." The Expert set gets into cheaper mercenaries but those are expected to take part in military campaigns and expeditions, not dungeon exploration.
In the couple of extended OD&D campaigns I played in, we'd have somewhere between 3 and 10 PCs in a given session (most commonly somewhere in the middle), and PCs would routinely have one or more hirelings each. Usually not more than one classed hireling per PC, but I saw a few exceptions. The smaller groups would normally at least double our numbers using hirelings if we could afford to, and the larger groups would still bolster our numbers, though the DM might outright put a cap on numbers for ease of play or discourage us from doubling say, 10 to 20 party members by warning of increased chances of us being heard coming and Surprised, and penalties or elimination of our ability to Surprise enemies.
In the Castle Greyhawk game I played in a typical session might have a roster and marching order something like:
1st rank: my fighter, another PC fighter, and a PC cleric, each with melee weapon and shield.
2nd rank: my fighter's henchman with shield & spear, our PC MU with a lantern, and a second fighter hireling with shield & spear.
3rd rank: Second PC Cleric, PC Thief, and the thief's MU henchman.
4th rank: zero level porter/muleskinner with torch leading a mule, fighter hireling rear guard, and the PC thief's Thief apprentice
For a total of six PCs, five classed and leveled hirelings/henchmen, and a zero level torchbearer/muleskinner/porter to help with treasure and light. 12 people and a mule.
Some sessions we'd be more numerous. Some sessions we might only have 6-8 total, but that'd be on the small side.
PC fighters would often also carry a spear to have the option to throw it, or to move to and fight from the second rank after we took a hit or two and it became excessively dangerous to be in the front rank.
This is one of those "everyone did it differently and nobody did it by RAW" areas of old D&D.
OD&D didn't have true multiclassing. Elves could pick being a fighter or a mage day to day, but everyone else was a single class. Multiclassing was partly the benefit of those new classes like paladin, ranger and bard.
Not exactly. OD&D as originally released in 1974 was as you describe, but as soon as Greyhawk arrived in 1975 it introduced multiclassing almost exactly as we see it in AD&D. Demihumans got the option to pick multiple classes at first level, got all the abilities (as opposed to switching from adventure to adventure as elves originally could) and from then on would split all XP evenly between all of their classes. Dwarves and halflings only got Fighter and Thief as options (but still more than 1974, which only allowed them to be Fighting-Men), but Elves got more options and the new Half Elf race got a BUNCH of options, including triple-classing.
OD&D includes a pretty darn vague rule on switching classes permanently ("Changing Character Class", Men & Magic p10), which only applied to "men", as Dwarves and Halflings could only be Fighting Men, and Elves had their own rule. This got expanded in much more detail in AD&D, ref "The Character with Two Classes", PH p33.
Paladin as it appeared in Greyhawk was an available upgrade to your Fighter if you had a min Cha of 17 and has always been Lawful since the creation of the character, which gave you special abilities but could be permanently lost if you ever committed a chaotic act. Ranger was introduced in The Strategic Review and was a strict upgrade over a Fighter, basically, though you had to meet minimum ability requirements and to be Lawful. Bards were another Strategic Review class and more like what you describe- being a mix of melee combatant and caster. But they didn't appear until after multiclassing.
OD&D went through a period where they had sort-of a version of the weird race-as-class thing those games did, but late in its run sort of unpacked it into multiclassing of a sort for most of the non-humans. Cleric was just never one of the options.
Multiclassing as Cleric was an option for Half Elves starting in 1975's Greyhawk, and that was retained in AD&D.