I actually once thought about the whole Mokou situation, and my thoughts are as follows. Under spellcard rules (which are ALWAYS active), danmaku may not be lethal, but still can damage things (characters' clothes) and hurt living beings (Mokou being in pain after the fight). There were battles with humans before, and they were okay after a few spellcards' worth of danmaku, but a full Extra boss battle might be way more dangerous, which is why Reimu was reluctant to fight at first. Based on this, I've always seen Mokou's "Resurrections" as just her getting knocked out but instantly regaining consciousness thanks to her ability.
Based on cuc's "Border of Duel" theory, there may be another explanation. Everyone fighting has a barrier, which explodes once opponent is knocked out or out of power. Mokou, being a human, possibly just gets knocked out after each spellcard, but her ability restores her stamina until she's too hurt to fight.
Also, I wouldn't take the talk about anyone dying in a danmaku duel seriously, no matter how it's worded. In EoSD, Remilia and Reimu talked like the latter just killed Sakuya, but she was very much alive in PCB.
I'm sure I know the answer to this already, but I still want a clarification: demons, or youkai, are not immortal, right? They're simply very hard to kill, but they can still die, right?
Yes, youkai have lifespans and can die, we know this much.
I think I may have miss something but where in the spell card rules is said/implied that the duelists are protected by some magical protection mechanism?
That's just a theory cuc worded earlier. It would explain a lot, actually.