- I sleep under the tarp, away from the edges, and don't pitch in low lying areas. Rain and snow have not been an issue at all.
- Yes to mice, I've had a bivy and shirt chewed up by them - which is entirely possible in a tent as well. I've also been woken up by spiders crawling across my face. When it's buggy, I add a lightweight bivy (all of the versions I've used were less than 8 ounces).
- I use a tarp, with or without a bivy, year round. Car camping, a tent is simpler and the weight doesn't matter, so that's the only time I use an actual tent.
Tarps I use vary from 12 ounces to 22 ounces, with stakes and rigging. A bivy adds 8 ounces (or less), or I swap that out for a groundsheet when I'm not using a bivy. A groundsheet can be 4-8 ounces. So total shelter weight varies from 16 to 30 ounces for me, with the heavier end of that representing a large flat tarp that I can pitch a multitude of ways in response to current wind and precipitation conditions.
Floorless can be a little more finnicky than a full tent, but in my opinion is more versatile. It's not for everyone though, like most things.