My go to shelter is a hammock - putting enough pad or underquilt underneath you is pretty much mandatory. I have been into the low 20F range without much trouble for me, other than shaking ice flakes off the underquilt. Pads have to be wide enough to keep the cold from seeping in the nylon; a short pad crosswise or a Speer pad extender (which they don't sell any more, but are fairly easy to make) helps.

I've had critters run over me while tarping. It just doesn't bother me much. The tarptents are mostly enclosed single walls and are a good compromise between tarping and tent - there are some nice models. I have a Sublite and the nylon version as well.

Never seen a snake get in with me, but have had various bugs crawl under the groundsheet to stay warm.