You'll be fine extending at immigration for the 30 days. The only warning I would give regarding immigration if its your first time is make sure you have everything they ask for as its often a long day and if they send you out to get more photocopies for example, you might have to wait longer when you go back in.
No. Anyone who leaves Thailand and tries to get back in constantly in short spaces of time are not classed as tourists. Anyone can leave Thailand and not go back to their own country and still be a tourist
I was talking directly to Michael and his specific concern. He is going back to his own country after 35 days so will obviously be classed as a tourist