- Since ghosts aren't youkai as per say (base on Reimu words in CoLA...), then what about half-ghost/phantom/evil spirit?
Half-ghosts seem to be considered half-youkai, going by Youmu's classification in IN. I don't really know why.
Phantoms and evil spirits are types of ghost, being former humans that died, so if ghosts aren't youkai, they aren't either.
- Does the stamina/energy of those immortal being (the one who consume the Hourai elixir) replenish to full when they die and resurrect?
Let suppose Mokou die to hunger somehow (in the event that she can even die), when she resurrect does her stamina also replenish or does the Hourai elixir prevent her from dying from something like extreme hunger?
They go back to being exactly as they were when they drunk the Elixir, IIRC, so yes. Mokou constantly starves herself to death (or at least near it), since she doesn't really care.
- Which vampire trait legend has already been confirmed in touhou? - Are Remilia and Flandre dead being for example or....
The only "traditional" vampire traits they have are burning under the sun, being unable to cross moving water, turning into bats and mist, and sleeping in coffins. It is unknown if they are undead or not.
Assuming that a Vampire can give birth with another vampire is their a specific name for this new entity? - a bit like Dhampir (human and vampire)
That's unknown, it was never discussed in canon.
- Has there been any explanation given provided in touhou canon as for why Yukari hibernate in winter?
Nope. Akyuu provides a theory that she spends the time she says she's sleeping/hibernating on the outside world, but it's just a theory.
I have a question; in Yuyuko and Yukari's profile for PCB, it's said that Yuyuko was the one that thinned the barrier between the Netherworld and Gensokyo, later requesting Yukari's help in closing it. Youmu and Yukari's profile on PMiSS, though, say that it was Yukari's doing. Do we consider this a retcon or just a "Akyuu lol" moment? 'Cause, many many things that are heavily implied as being Yukari's doing (the creation of the barrier, the original spellcards rules, etc) don't directly state her as being the instigator, yet in this case in particular, two different articles (maybe more, I'm not sure) explicitly point her as the culprit. Maybe this alone should indicate that she's
not the culprit, but...