Yes, he does; returning to play counts as arriving. Also, when a unit card leaves and then re-enters play, it’s considered to be a completely different unit from before.
Related behavior: using Max Geiger’s maxband on a unit with Forecast will send it to the future zone.
Can I target a Tech II Illusion or an Illusion with ATK>2 with Forgotten Fighter or do I also have to follow the conditions to play spells in connection with Illusions?
No, target restrictions still apply. Compare to Thunderclap, which can target units/illusions not in the patrol zone, even though it wouldn’t usually have an effect.
And it’s a stupid card and thunderclap definitely should be changed to only target patrollers and I’M NOT SALTY
That´s what I thought. And another question:
If a Reteller of Thruths dies as the first Illusion in a turn, does its ability still tigger for a second dying illusion in this turn? I guess not, but I´m not sure…
No, it’s no longer in play so there’s no ability to trigger. Unless they die at the same time.
And can I target an Illusion with Hallucination? It is an Illusion already, so if I can do it: why? Doesn´t the ability have to be able to resolve?
Nope. In fact, I think the main purpose of Hallucination, in MonoBlue, is specifically to pop enemy illusions, usually by hitting normal units with two Hallucinations in the same turn, or having Dreamscape in play.
Its main purpose is more likely to make friendly non illusions benefit from macciatus, but yea, hallucination, as a single card, does absolutely nothing. A completely pur combo card. I would challenge you to identify any other card in codex which REQUIRES another card be in play or be played in order to have any effect whatsoever. Plenty of cards are stronger when combined, but hallucination legit does nothing by itself.
Time Spiral
There are lots of cards that work with Time Spiral, and you can reliably use Prynn.
Ehhh, 2 Heroes can benefit from casting that spell directly. I concede the point, but offer time spiral is far more easy to combo, including with heroes and tech 0s, any one of which is always available to combo with it. Hallucination just pisses me off in its extremely narrow design.
So what is the explanation that Hallucination cannot target an Illusion? Because it has no effect in this case whatsoever? I would like to know the rules mechanic to understand it? Thanks in advance 
You can play spells that have no legal targets, and you can play spells that can’t perform some or all of their effects. E.g. you could cast Hallucination where there are no units in play and trigger Harmony to get a Dancer token, or you could cast Martial Mastery with no cards in hand, fail to discard a card, and then draw 2 cards.
Oh, it can target Illusions. I meant “Nope” in response to whether the ability has to be able to resolve. Sorry, I just realised that reply was ambiguous.
I see. But now I´m even more confused: I always thought that you can only play a spell or ability if at last a part of it is resovable. Where can I find the hint that you can also play them without legal targets?
So the entry in the unofficial manual rewrite:
“[…] If a spell or ability tells you to do multiple things, you can play it as long as you can do one of those things.”
is wrong then, right?
I can play a spell or ability just to for a meta-effect. Is that correct?
Yeah, if that’s the case then the rewrite is wrong. Generally speaking, Codex works on “do as much as you can”, and there’s nothing stopping that from being “do nothing”.
Another question that, if Nekoatl is right, I also had wrong in my memory: If I go to max level with Max Geiger and I trash a unit with Forecast that is already in play: does it return to play with its “arriving” runes again? I always thought that “return to play” means that it will return to play and not into the future zone?
It returns as a new instance, so it has to be forecast again.
So the future zone is also “in play”? Oh man, I was learning some things wrong 