1. I would say that low-mid 70s is on the low side. I try to not let any of mine drop below 75, but better nearer 80.
Most of the time you just gotta set it up and see what happens before you need to fix anything. I have enclosures with no trouble, others I have had to do a little something here or there to fix the temps. You should be setting up the enclosure prior to purchasing a snake anyway, so it is not difficult to check all your temps previous.
2. No idea. I've heard of bottle caps.
3. The 27 qt is probably good for a while, but it is hard to say. Search for some threads on the subject, I know there are several.
4. The tub lids should have the locking ends that go over the lid. If it is a juvi, those are going to be fine. If it is not the type of container that locks shut like that, or the bp is much bigger, yes, you are going to want something to ensure it is sealed properly.
5. I used a knife, I'm not sure if one would save you any large chunk of time over the other.
6. ? (Where I live the cheapest males from breeders are $50....stores are well over a hundred usually).
Good luck.