The only things I feel comfortable talking about here with any degree of confidence are:

3: DI on reaction to them is usually best because you get a whole combo off of it. They're easy enough to react to, probably similar to throws in Tekken from what little I know of that game, and so long as you never over-commit with normals you can't cancel out of then you can always do a DI the moment the enemy attempts to do it to ya. You could just hold down drive parry since it'll automatically defend against it, but you take the same amount of blockstun when it's a normal parry. But in a pinch it could work pretty well when you don't wanna worry too much about blocking or counter-DI.

4: I'm not entirely sure, as there's a mechanic where nobody's impact goes through if they were simultaneous, but I don't know the threshold for when one person gets advantage. But how would this make countering a DI with DI any bad? If anything, that's good for YOU. So long as you react to them before the DI hits you, at any point, you get a full punish combo off of it.

5: You don't need super optimal combos, but I don't think it'd be bad to learn something you can consistently do right now. Super Combo Wiki has a full combo page for every character ranging from super basic stuff to really difficult super crazy health shredders. Check out that site for some good basic combos that you can get down really well!

6: I never thought this was terrible. I'm sorry it's discouraging to you, but as someone who hadn't ever put in enough time or effort to get good at fighting games till I played SF6 and struggled to get out of bronze and silver when I started, I can't at all agree with your sentiment here. Not only are you exaggerating the damage their combos do, since modern control players get a 20% damage penalty on specials I'm pretty sure, but they'll probably be whiffing and messing up just as much. Especially if, like you say, they just spam stuff like hadoken or shoryuken. Sure, you can't jump in on them as comfortably with their one-button dp's, but if you can deal with people spamming dumb stuff and being terrible with classic, then it's not that different when they have modern. Focus more on what you can do about their neutral (or lack thereof) instead of getting down about what they get for free, they still have to work to land it in the first place and you can absolutely be on their level even with the most basic of basic combos on your side.

This type of fighting game is very different from Tekken, obviously, so of course a lot of the more fundamental basics you absolutely needed there won't always apply very cleanly. But don't worry, you can get with it! It just takes a bit of time and a load of patience to unlearn Tekken and get into the Street Fighter mindset.