For me it very much depends on whether the target is otherwise engaged in melee. If yes, it's pretty easy for a Thief to disengage, hide for a round, then strike again. And I've always had it that a caster is pretty much unaware of anything while in process of casting, meaning they're easy pickin's if the Thief can get close enough.
These are house rules to make the Thief better. In 1E BtB you usually need to have Surprise (see Gary's notes on DMG p19 re: negating the attack form).
Which is fair, except then they broke it by allowing ranged sneak attacks.
Eh. Sniping is very much in flavor for assassins (and for Thieves making this sort of attack). B/X always allowed that, going back to 1981 (it just has to be an attack while "unnoticed from behind", and Thieves, who can use all weapons and point-swap to increase Dex, frequently make heavy use of ranged weapons). 1E isn't explicit on it being a melee-only attack, but does imply it by calling it "back stabbing" and only calling out melee weapons as examples in the description on PH p27.
Too much so IMO, eventually leading to 4e where Rogues became the primary damage dealers (which should be the Fighters' job!) and Fighters became defensive tanks (which should also be their job).
"Should" being a matter of opinion. If you want characters to have clear jobs in a fight (which was part of 4E's intent, and indeed how other editions worked, 4E was just better at explaining it and more consciously designed to build on that), then making the "glass cannon" or "artillery" role a thing for rogues and rangers is a reasonable choice. Of course, the roles weren't straightjackets either, and you could build a fighter to be more focused on damage and less on tanking.
The Thief was almost always the party's scout and trap-finder and ideally the DM would reward such activities with some xp now and then. That said, I've always suspected the reason for Thieves needing fewer xp to advance is that the expectation was that they wouldn't get into combat unless they had to, and thus would get less xp than most other classes.
By the book "All surviving characters who took part (no matter how insignificantly) in slaying the monsters" get an equal share of monster XP. Of course monster XP is generally a small fraction of treasure XP, which the players/PCs divide as they choose. DMG p85. I do like awarding individual xp for heroic/clever individual actions, but that's a house rule in 1E and an optional rule in 2E.
Fighter-Thief is a good one; you get access to all the nice Fightery tinker toys and then can use them to backstrike, though at cost of being restricted to very light or no armour in order to be able to sneak. I've seen some Thieves go the MU route for armour (bracers-ring of protection-dexterity) and it's worked out pretty well.
Yeah, multi-classing Thieves was a popular option to make them better, although of course then your small faster advancement advantage goes out the window.
I have never in my lifetime used a rulebook guideline on what magic items to give out. I do what I think makes sense for the adventure and for the party. .
I think the vast majority of us did that!

But one of the things text analysis in the OSR taught me is that the tables are set up in ways which are advantageous to certain classes, and that's part of the design. The way my groups played AD&D back in the day, for example, Clerics found usable magic weapons more often than was probably the intent, and Fighters DEFINITELY found badass intelligent magic swords less often than they were supposed to. And as a result Fighters were weaker in our games than they should have been and had fewer cool abilities.
Henchmen survived slightly less long than an unnamed marine in a xenomorph attack.
No matter how many characters set out, it very quickly became equal to the number of players.
Highly variable table to table. In my last (three year) old school campaign the two parties initially went through hirelings and retainers at a pretty high rate. And since those characters only get a half share of XP their advancement was slow. But once a couple of retainers survived longer and gained a level or two, their death rate dropped off a cliff. The players would use them more cautiously as they got more valuable and competent. For ease of play I capped my groups at having as many retainers & henchmen in the party as there were actual PCs. Or they'd have wound up with more.