Is there a reason for Shirou constantly being in pain? Is it because the Caster card is not compatible with him? Is it to introduce some token grimdark stuff?
1. For the first one, what exactly could Shirou offer? Would Shirou be living in Uminari with Nanoha there or something?
2. Your second idea makes me imagine Shirou somehow summoning Hayate, except this Shirou is the one in the TSAB with Nanoha and co so who is the Hayate as Caster he summons?