Honestly, as far as the Tyrells having Loras as the future of their house, it always confused me - a large, old house like theirs would have more people, and even if the main branch was at a small ebb, they'd have cadet branches of the House aplenty. A bit like the Karstarks - if the Starks all died, a Karstark or someone could claim descent from them, continue on being Starks proper, etc.