The astral temple is created by the imagination so even if you’re imagining it it’s still there. It’s the astral the collective unconscious/realm of thought. However yes you can make portals to other layers and parts of the astral plane as a whole but not the astral to say the etheric or other spiritual planes.
No because it only really takes someone who is in control of their mind because in the astral if not you can easily manifest your fears as well as your desires and such.
the infernal nation exist within the etheric plane, you can transition from the astral to the etheric, etc but a portal isn’t needed for that. As you’re unlikely to be allowed to have one that extends into her territory like that in the first place, think of it like this, the infernal is a nation, lilith’s place is within the infernal, you’re a foreign individual, you wouldn’t be allowed to just ‘portal into’ their territory they still have common laws for citizens and noncitizens who enter their nation.
but no since the astral is the realm of thought the portals are only really useful there as they are not created by energy work but by imagination.
However, since you’ve never been you’d need to evoke in your astral temple and have her bring you, as she is very capable of taking you and transitioning you from astral to etheric.
Or you can just ask Lilith to establish a portal to her realm for you in your astral temple, in a similar way EA has portals in his physical temple that lead to various realms that were opened by the spirits themselves for him (the big black mirror you will sometimes see behind him in his videos is one, for example).
Which reminds me of a yet another question, do I have to do a formal evocation to have Lilith or any other deity enter my temple, or can I just request their presence in the temple?
You would still have to summon them. As previously stated, the spirits reside on the planes above the astral so they have to come down. Most magicians perform the same rituals in the astral temple that they perform in their physical temple, but evocation is much easier.