1.I have long arm and narrow(thin?) chest and terrible at make arch.
Does it have to do with the fact that I bench slightly more with close grip? (I feel more comfortable with my elbow tucked on my side.)
2.in Bridge, I guess close grip bench is supposed to be accessory lift. But in my case I lift slightly more with close grip AND I think my wide grip bench is not low enough to be accessory work.(maybe 5% less) what would you recommend me to do?
I think it’s not unusual for people to not be very good at arching when they first start lifting, so keep working at it. You’ll get better.
It’s fine to use a closer grip.
Nothing really. Keep running the program as it is. If your preferred grip for bench press is narrower, I’d want to know how narrow this is compared to your wide grip bench, what you’re benching, and how long you’ve been benching. That said, it doesn’t really matter what you choose as long as you’re consistent and get the training in.