1. ST was written as a novel to make money. Anything interesting about it is a reflection of Heinlein's philosophy that if he was interested in something, somebody else probably would be to and therefore would buy his book. Characters do not reflect the author's views on anything, they reflect the characters views. It wouldn't have been at all realistic to have book where everyone thought it was the best idea ever and signed up and as such the book probably would not have done as well. As for Heinlein himself, his only comment when asked about it was that he thought the society would probably do very well in the right circumstances, but that without experimental confirmation of at least a few generations it was just an opinion.
2. I fail to see how this matters to the premise of Starship Troopers. Our currently military is nothing like the the one in the book, in terms of technology, structure, doctrine, etc., nor is the background society very similar.
3. Again, fail to see how this matters. Starship Troopers is 100% volunteer and will take anyone, as long as they can understand the oath. No military anywhere ever has been designed like that.
4. No... volunteer force. You can't possibly argue that the "left" implemented the draft and then argue that a 100% volunteer force would be an "extreme left" idea. No. In fact the book is roundly criticized in a number of circles for being unrealistically libertarian, which is about as far "right" as it gets.
5. Again, not relevant. Starship Troopers is a 100% volunteer force. No drafts. No possibility of a draft ever happening.