Between loading screens, the requirement for you to be hanging over the device and not get distracted, and the missions themselves, I suspect that harder missions pretty much never take so much longer that they're not more worthwhile. As long as your team will reliably not die you should do the hardest one you can.
Probably. Enemies take about two hits on level 4. I wonder what the actual drop rates are.
I'm a Captain Marvel fan so getting her leveled up has been my priority but man, the bio crawl for 5* is depressing. I've managed to get a few from Enemy Shifters but that three per day from the Arena is not enough. I've only seen bios for her in the Shop once.
Three guaranteed bios per day was great when I first started. She's lagging behind some of my other characters now, though, at rank 5. She has shown up in the shop a few times but it's hard to pull the trigger on bios when they're only going to get you like 15% of the way to the next rank.