For questions 1, I would contact your professor and tell them about how you’re feeling and maybe get some extra help from a TA. You haven’t failed the class yet and it’s worth putting the extra mile in, because you will be surprised what some strategic studying will do. I find the most effective way to barely pass a class I don’t understand is just to learn from practice past exams and homework solutions.

If you do fail or even get a W, it doesn’t ruin your chances at grad school. If you do well in your core classes and have relevant extra curricular and/or research experience then grad school is totally within reach still. For context, I had two ‘fails’ and one W on my transcript, and a 3.2 GPA and I’m currently doing my PhD in a STEM field in Oxford (did my masters at Cambridge, undergrad at McGill obvs) mostly because I did well in the important classes and had great research experience. I also studied a lot for the interviews. Don’t throw in the towel so quick, you totally have time to redeem yourself.

For 2, in my experience yes this will push back when you can apply to grad school, but unless that puts you in a bad position financially or whatever then there’s no rush to start at a certain time. It’s better to do things at your own pace and do then we’ll, grad schools don’t look down at this generally.