It doesn't have to stack. It replaces. When at zero hit points, the five from the maximum hit points is gone. If you're using it to get people back up from zero, it works fine. Sure, saving a 1st level slot isn't bad.
Expect table variation there.
Mind you, I'd also expect table variation on what happens to someone with an aid spell on them, takes damage, then has the aid dispelled.
At lower level the cleric hides. As we get higher, the cleric has more hit points and can stay in melee range.
Honestly, we found the opposite to be true. At low level, the cleric is as tough or tougher as anyone else, and the monsters have fewer options. At higher level, the monsters have more mobility, more damage, more AE, etc.
Clerics obviously don't always heal. The reality is we don't always do things one way. I imagine your (and every other) group is the same. It's a matter of assessing what needs to be done at a certain point in time that is most important.
Ayep.
Your party must be quite large to hold multiple tactical points. We can manage two at the most with five unless we're using a wall spell, then maybe three if we seal one off with a wall or similar impediment.
Not really - varying number of people show up. Always at least 3, sometimes 4, sometimes 5. We just had a player change, so I suspect 5 and 6 will be more normal rather than the 4 that was more normal before.
But, really, that's kinda the point. The party is stretched thin as a general rule, with more things to do, but fewer raw numbers. We tried the whole "fight the entire orc tribe of 40+ orcs at once" thing, but honestly we found that the game gets really really boring after a couple rounds of that. Your mileage clearly varies.
Lower level encounters allow for less combat healing.
Again, experience differs. We had to use more combat healing at low level than high level.
Unless you're using a higher level spell slot. Then the healing gap is quite a bit worse. You seem to be using healing word at 1st level all the time using weeble-wobble heals. Our DM killed that tactic. Unless you do use it with higher level slots.
The difference is 2 per spell level. So, when I say the difference is fairly minor I really do mean over the entire course of the spell. 4 or 6 hp difference is less than a single hit's difference. Depending on the type of combat, that might round in your favor, and it might not.
And, to reiterate, I'm just talking about the spell Cure Wounds. Aid and Mass Cure Wounds are different stories. If you gave Cure Wounds just slightly more oomph, I'd even be on board with it, but otherwise _for most groups_, it's misleading advice that could get them in trouble.
The most trouble I've ever seen groups have with D&D is when they overfocused on defense and healing, and just couldn't take care of threats. Attrition starts out in favor of the PCs, but numbers and hp pools almost always go to the monsters. Your group is not in that boat, but many others get there.
I design dragons specifically to be epic fights meaning they will last as in not die by the third round. If you have a party capable of outputting 500 points a round, you will fight a dragon with 5000 hit points.
I've been there (see the work I did for Epic 4E LFR), but that's not a stance I feel works in 5E. Give the characters more interesting options and I can be on board, but otherwise it just devolves to mathing the time away for me.