Based on flourishes with Beast Medium, I think you could expect to get 30k for the snarble, give or take 10k cr.

Since it's a 2* item, you can unbind and THEN upgrade, but that's mostly not going to matter. The one thing you don't want to do is upgrade and then unbind.

I wouldn't keep it if you already have a BTB; if you're going to get an item you already have, you're usually going to want a bigger increase than just medium. And if you're just getting to 5*, you've got a lot of stuff you'll want to get instead before you start upgrading to UVs. Side note, I would recommend getting at least medium UVs on all of your starter 5* gear since medium UVs don't cost that much to buy off the Auction House.

And yes, weapons with an effective damage type are the ones which want damage bonuses the most, since it's a percentage modification and effective damage yields a much higher base value. So a Piercing weapon with vs Beast or vs Fiend UVs is good, vs Undead or vs Gremlin is meh, and vs Construct or vs Slime is downright useless.