Intellect Devourer magically consumes the brain of the victim. Once it leaves, there's just an empty hole where the brain used to be.
To quote the monster description:
the intellect devourer magically consumes the target’s brain
Death Ward, on the other hand, prevents the subject from dropping to 0 HP - it drops to 1 HP instead. So while under DW, the Intellect Devourer is actually protected from ejection, if it indeed gets inside in the first place.
Once it leaves the brain has to be grown back in one round, or the subject dies.
"The body then dies, unless its brain is restored within 1 round."
I was about to write how DW would not protect the victim from dying because of a lack of a brain, but then it struck me that as a DM, I would probably rule that "magically removing someone's brain" would fall into the "effect that would kill it instantly" -category which DW protects the subject from.
Mostly because, while the body is still living under the Intellect Devourer's control, there's nothing of the victim left; no memories, intellect, etc.
(Which opens a whole new can of worms in the form of "brain vs soul" -discussion which is open to many interpretations...)
So how DW works in this case (without finding any official ruling on the matter saying otherwise), in my opinion, would be that the Intellect Devourer could not remove a target's brain and enter it, if the target was under DW's protection (which would then end DW and leave the target open for another try, later on).
This is the danger in using homebrew material. The creator does not typically have the benefit of an editorial staff or design team to truly vet the material before play. Also, the playtesting sample size is so small as to be almost non-existent. Because of this, homebrew material often requires on-the-fly modification or adjudication.
We can still apply some basic rules, although it would ultimately have to come down to DM adjudication. Let us proceed under the assumptions of the homebrew. There is nothing in the description of the Plant Wild Shape ability to suggest it works differently than ordinary Wild Shape upon the shape being brought to 0 HP. In the absence of specific, we must assume general.
When the gas spore drops to 0 HP two things must happen:
1) the gas spore explodes into spores. It is not only dead, it is gone. It has ceased to exist.
2) The druid reverts to humanoid form, taking whatever leftover damage might be involved.
If the gas spore disappears first, there is nothing left to turn back into a druid. If the gas spore reverts to druid form, there is nothing left that can explode. It's a binary choice.
Since the reversion to druid rule of Wild Shape normally circumvents any rules about death (the beast forms get no death saves and do not die), we should assume that the gas spore's explosion ability should come second. This is further reinforced by the fact that it would be irretrievable character death otherwise (no body), and no fun to play.
The most playable answer would be to have the spore revert to druid without triggering the explosion. The lesson here is to be very careful with homebrew and don't be surprised or disappointed if you run into problems when interacting with the official rules.
Best Answer
There is absolutely no precedent for this in any other part of the game system. We are definitely in DM ruling territory.
However, I would say these are no more your character than if the story involved you creating progeny any other way. The only difference is the speed in which this is possible. The new gas spores are your character's "children". Sadly they are mindless, highly dangerous, monstrous children, and are unlikely to recognise or respect their relationship to your character as their parent.
Probably the only useful thing they would do is ignore the druid if he/she was currently transformed into something that they could not infect, such as another gas spore.