1- Are they only suggested for cars that are driven hard/raced, or would you recommend for my weekend only, gently driven, Mustang?
No, I would definitely get one. I've only gone racing twice and drive spirited a few times a week but I wish I did it a long time ago. My car has 24.5K on it and my manifold was gross. I just installed mine this week.
2- Which brand is best? I've seen references to JLT,UPR, & Moroso.
Any of those will do.
3- How many catch cans to install? Only JLT seems to suggest possibly using (2) cans, one for each side of the engine, & says that the passenger side is more important.
Unless you're racing every weekend, just the passenger side is fine.
4- Are there any issues with Ford warranty? I asked my dealer about this & whether they would install the catch cans, but can't get an answer.
No. And don't pay them to install it. It takes longer to take the decorative cover off than to install the catch can.
5- I realize each state is different, but is there any reason to think these devices could pose problems with emissions testing (I'm in NJ)? I did see one post where someone thought they caused an issue with California emissions testing.
It's technically an emissions violation in some really heavily restricted areas. Just double check to make sure they don't pop your hood and look for that stuff. If PVC systems are not mentioned, then don't worry about it. But then again, I can't imagine someone would really know the difference unless your emissions are done at local shops or something like that and know what to look for. I'd be worried about this much 0.0000000001/100000000000. Just keep the factory hose.
6- Any maintenance required? Do they just screw off to empty?
Screw it off. Empty it. Screw it back on.