I know that feeling, you start to get so used to that way of living that you don't realise it's absolutely not normal. The way I look at it, if I placed my dad how he is now infront of a version of myself 2 years ago, I would certainly notice the difference. Write and date things down, as it can be helpful to see how much he's developed - I have a list from a year ago and the difference is crazy.
Yes thats exactly it, they don't realise how much you bend backwards in order to keep them from doing anything thats challenging and ultimately keep the peace. It's only when they find something that you didn't foresee, when it becomes a real game. You will hopefully adjust and find yourself not arguing back eventually - I certainly don't half as much as I used to. But don't feel bad if you do, it can feel like they're trying to test you sometimes, and its only natural to want to bite back. If you can, take a step back, go into a different room, take 5 to cool off. I know it's probably the hardest thing when they blame you for things you haven't even done - but trying to persuade them of that often gets you nowhere, and they can argue until the cows come home. I find my dad even contradicts himself and bends the argument in all different directions just for the sake of 'being right'
The hardest thing is that there is no space for you, for your mental health, and for you to have a bad day. They don't understand it, they have no awareness of you and how you might be feeling. It's very isolating. I'm lucky in that I live with my mum and boyfriend, so we are sharing the load, but I can imagine being on your own is a HELL of a lot harder. If you feel you can't help him anymore, never feel bad for asking for help. They will never admit they need it, but you certainly can admit it for yourself. Ultimately, you're going to have confrontation either way, so why not get some help in, or a chance at respite so you can recharge. Maybe not now, but down the line you definitely need to. (I know, so much easier said than done. It irks me when people tell me to do it but they are right!)
The road is certainly daunting - I have exactly the same dread. They aren't the same person, you probably feel resentful like I do, and you can't even make the most of time together because it's such a stressful and depressing time for you. I hope knowing someone feels exactly the same helps you, and if you ever need to rant, I'm happy to hear! Please take care of yourself. You will have better days.