All of that is pretty Easy peezy sadly.
First of all, for anything Boost kick related I recommend this video by Trifroze.
https://youtu.be/WQD6fPQXYwE
He covers everything you'll ever need to know about that move
To DI dsmash into downB, there is no way to avoid it entirely. You can only hope to make the ZSS player miss the spike. To do that, I usually recommend DI'ing into ZSS. That'll make you take the brunt of the hit instead of the spike.. If you're at high percents you'll likely die but oh well. A really good ZSS player may be good enough to read that you'll DI in and can adjust but you'll almost never ever meet a ZSS player that plans for the person to know how to DI..
Her grab combos are something in themselves. First of all, how are you getting grabbed? Just spam rolls and we'll never catch you thanks to sakurais nerfs. But anyways if you get caught then mix up your DI between DI in and DI away. If the ZSS player is good they can react to your DI no matter what, in that case, you may wanna prepare for boost kick..
Trifroze covers this but I'll explain something here that nobody seems to fully understand..
When you are stuck in Boost kick, you have two options to try and survive.
You can either try and smash DI out or you can DI to survive the hit. You will likely not be able to try both.
If you are at high percents... Try your absolute hardest to smash DI out. Since you'll probably won't be able to DI good enough.
If you are at low percents, DO NOT SMASH DI. This is why you see people dying off the top before the move finishes at like 30 percent...
ZSS with rage is scary in that moment. Just DI down and take the hit. It'll keep you away from the top blast zone and you won't die (unless you're literally right beside the side blastzones obv)
I cringe everytime I see someone Die at 30 and rage at ZSS being broken because of this simple fact. People tend to only ever pick one option when they get hit by boost kick.. Eithe SDI or survival DI. And if you do the wrong one you'll die early as hell.
Also finally, ZSS has no real true 50/50 kill setups that I've seen be reliable. People say dthrow up air is a 50/50 but as long as you DI away at kill percents I'm pretty sure you can always jump in time.
Any more questions just let me know.
Peace
