As far as meats, 1/2 lb per person is a pretty good rule of thumb. I disagree that most people will only eat one entree though, most people will try a little bit of everything, so if you make half the amount of each, you'll probably be fine.
Potato salad, I usually do one potato per person, but kids don't usually count. You can subtract a few potatoes if you feel like that's too much, but that's usually how I do it.
You can absolutely cook the quiche and warm it up, I do that all the time. Or just cook it 1/2 way and do the rest the next day, either way works. If you make banana bread a few days early you won't necessarily need to freeze it. I'd wrap it really well, don't cut it and stick it in the fridge, it will be fine.
For salad, do less than you think. People don't take a lot of salad. They usually take some but it looks so big on a plate with lots of other stuff so I always have salad left over. Plus, if everything else looks too good, salad is the first thing most people will give up. I would seriously probably only do one head of lettuce plus extra veggies, etc. If you run out, oh well!
I think people munch on little desserts so if you do cookies I'd do a couple dozen easily. Kids are drawn to them! People are also always willing to take desserts home, so I feel having extra is better.
If I were you, I'd do a few quiche and a ham, skip the crock pot. Add in the sides you mentioned and I think you're good. I would add a fresh fruit platter because you can mostly get kids to eat that , it's easy and looks pretty.
Also, just FYI, Costco does a cross cake that I got for my son's Christening and it was so delicious and cheap! It will easily feed all your guests with leftovers and only costs $18. It looks a little funeral-y in the picture at the store, but if you get it without some of the weird scrolly things, it's perfect for a baptism!
I host lots of parties and holidays so I hope this helps!