1.) I don't really like boaphiles, but I have several AP cages and I love them. Sure I think every cage could have improvements though.
2.) I'm sure you can make due with this, but if you start dealing with a female that weighs a lot, she won't have a lot of space, so it's more of a personal judgement call for you.
3.) Both options are great, though putting the flexwatt in the middle if you plan on splitting will work best, however with one snake that's not going to give them large place to regulate, so it should be on one side. RHPs work similar to CHE's, however they don't get as hot as they would instantly burn you. Here is a good read on RHP: http://www.reptilebasics.com/radiant-faq