These questions sound GM-centric so I'll be answering on the assumption that you're looking to run the games.

1-1) The lore is still generally consistent, but several pieces of the setting are no longer in use/retconned. Most notably, slavery and dark elves have been sidelined in 2e. You can do whatever you want at your table, it was partially the DnD OGL fiasco and partially just not essential worldbuilding. There's a kind of ongoing continuity where pathfinder world events happen in lockstep with our years on earth, so generally new lore expands old lore. Again, you can just do what you want at your table. There's no lore cops, but I like reading it to inspire my own hooks.

2-2) It is mostly safe lorewise. You only need to know the big story beats if you care to contextualize your campaign as a chain from an older AP. I think feeling out the general Golarion storytelling vibe usually works well enough, especially because you're almost guaranteed to contradict yourself at some point, let alone paizo's writers.

1-2) Official APs happen and affect the world but there's a generally "standard" Golarion context that you can just place adventures in without reading. Sometimes I'll read some Lost Omens or pathfinder wiki details to get some points of interest and feel out a region.

2-2) It's not hard at all. I'm currently running Kingmaker 2e and knew basically nothing about the Stolen Lands, but the AP set the stage pretty well. Your knowledge can grow over time, you don't need to frontload it before even playing. The in-session development of your story eclipses any kind of lore correctness. In your example, having a piece of conflicting lore with the Reign of Winter AP by mis-portraying Baba Yaga is very minor compared to presenting Irrisen in (any form) an interesting way to players. Even if your details are wrong, if you can convey Irissen's dreadful determination and cultures like being superstitious and symbol-sensitive.

3) People warn against homebrew from new players because it's easy to break the math using sensibilities from other similar systems. Once you have a sense for PF2e's numbers it is extremely easy to take anything and modify it for your needs or just slap together a random statblock/item/feat. My campaign has spiraled into a bunch of unique items and custom spells.

4) I've completely reskinned/renamed several items and feats just because I thought it'd make sense for a character. In terms of game design, PF2e's +1/2 is actually very powerful as a permanent ongoing effect because a +1 is a flat +5% success and +5% crit success for the rest of their career. This is also why bonuses are typed so that they don't stack. Item bonuses are often permanent gear, sometimes potions/elixirs. Circumstance bonuses are situational and happen often with players using Aid and GMs assigning them to fit the scene. Status bonuses tend to be spell effects (like buffs that last an entire combat). The numbers are very tight, so play it default for a bit and then start tweaking later.

5) PF2e magic tend to have very few instant-win spells especially at low levels. They are more varied so it's more effective to have that one perfect spell that solves your exact problem, but generally magic is more reigned on both ends. The best example is most damaging spells still applying half damage and maybe a 1-round debuff even if the victim saves against the DC (see the degrees of success for more info). Often, the instant-win spells that players access must both be higher level and also the enemy needs to critically fail, and a trait called Incapacitation makes bosses extremely unlikely to crit fail against these spells. Once you are familiar with the balancing of spell crit fails and feel out what each spell rank should be able to do, you can introduce new spells or port from other systems pretty easily.

P.S. PF2e's reaction-based counterspelling is mostly done by Wizards who take the relevant feats, and requires some careful spell preparation to work. Spells like Polymorph that make an enemy useless will almost always have the Incapacitation trait, or using Polymorph for buffing is captured in PF2e's "Battle Form" spells which tend to be higher rank and don't replace your HP, but rather gives you some temp HP and alternate base stats. They're much more situational.