Whenever someone shares a planet, sometimes for building specific structures, it seems there's no way of telling what structures one can build without it turning useless.

My suggestion is to allow any structures built on it by the legion to function fully regardless of the technology researched by the person who shared the planet. And then if the structures cannot be supported by the planet due to missions not yet completed or the technology not available to the sharer - make it functional for only a limited period of time.

OR

Allow the sharer to accept or reject the structures before they can be built.

OR

Just not allow structures to be built if they can't be supported.

+1 to this idea.
Also, I wish it was possible to use artifact buildings on legion members' planets. I have a good number of them burning a hole in my cargo (because I have already filled my planets with them) and a few friends could really use some.
As for the rarer mission and NPC buildings, I do think those should not follow my suggestion, as you get so little of them you won't fill up planets that quickly (or at all).

Option 1 makes the tech tree pointless. Option 2 would be overly complicated. I think option 3 is the only reasonable one.

It wouldn't make it pointless. My suggestion is to make them functional for only a limited time - at most one day. Then they have to be demolished or be rendered useless if the owner of the planet doesn't have the technology.

One reason I think it would be a good idea is to have a smaller legion a chance to be able to defend their planets with the use of structures built by their higher level legion members. On the other hand, large legions just let as many members as they could guard the newly invaded planets giving enormous amounts of attack and defense to it. I think there probably should be a limit to it.

Well, I don't think would really be necessary to make it temp. functional for your 1st suggested option to be viable. Might even make it less desireable to 'help' a friend by spending huge amounts of creds on stuff that will just get demoed 24h later. All that would probably really be needed is a way to differentiate between the stuff your allies put in and stuff that was already there to begin with and should remain too advanced for you. That would even open the possibility of encouraging friends to help builf if for example the Graviton Minefields built by my Konqul buddies had bonus attack.

It is NECESSARY to make it temp as that is the very idea for option one, otherwise, it would actually make Planetary Structures research pointless for some. Whether it is desirable or not to help depends on the helpers capability. It is simple really.

option 3 looks good...also if you want to let people know...put one of the ones you need help with up....that way they can tell your tech level