by the reasoning you stated, they could be affected by curses/manipulation that affect the space around them and not them directly. the same works for positive things. like if sakuya and mokou were in the same party, sakuya could slow time for her team as it is effecting time/space and not mokou's body.
as for shapeshifting, that would most likely be dependent on the youkai in question and the method of shapeshifting. for example, if reisen became immortal she could still probably make you hallucinate her into whatever she wants.
Ah yes. Illusions could still work. Nue's ability to hide something's true form would also still be effective, since it is an illusion, not her actually changing. And apparently it was just an observation of Kimiko regarding the change part. Though, it is said it makes one immune to direct manipulation, but now I start to wonder if youkai shapeshifting really counts as manipulation. Shapeshifting is a natural ability for almost all youkai, so maybe they intend the word 'manipulation' as using curses and manipulating magic on someone (like Yuyuko's ability for example). And, to a certain extent, the little girl disguises of the youkai already are illusions to a certain extent. I'll just assume youkai shapeshifting and illusions are still possible, but other forms of shapeshifting might be harder.
Extended life-span does not equal immortality.
What he said. The majority of youkai have a long but still limited life-span. Only the powerful ones can be rendered incapable of dying through age (Byakuren, likely Yuuka, Yukari and Utsuho too, maybe the devas) though it depends on race too (Remilia, Flandre). That still doesn't mean they're truly incapable of dying. Youkai can regenerate from most fatal injuries so conventional weaponry is ineffective against them, but powerful magic/spiritual energy and enchanted weaponry can perma-kill them. If a youkai is tough enough they can take a few blows and fix themselves up with rest eventually thanks to their regeneration, but if the blow is powerful enough it's game over. Yukari and other reality manipulating youkai could perhaps achieve some form of true immortality though.
Though, I think I read somewhere that the goal of the majority of youkai is to achieve immortality, which indeed makes it odd why none of the youkai party members attempted eating Mokou's liver. They might have excuses for it though. Remilia is already quite hard to kill, and might think Hourai immortality would be redundant, Yukari might already be immortal, Yuyuko is, well, DEAD, and Alice might just not be interested in immortality.
Maybe true youkai immortality is possible in a way that doesn't require Hourai Elixir, but oh well.
This all does create a paradox though. Youkai are stated to be capable of dying of age eventually, but powerful ones can overcome that age limit. Youkai also grow more powerful and wiser as they grow older. So, doesn't that mean a youkai automatically becomes semi-immortal eventually? Or is a spell/skill required? Why would they talk about an age limit if an old enough youkai becomes immortal eventually anyway? So confusing....
