Cheatman97, you don't need to apologize, it is indeed complicated but it is not your fault
All I know is that keys, regardless where they have been purchased, don't work immediately on other game selling platforms.
A key purchased with Uplay needs to be registered within Uplay, and a key purchased with Steam needs to be registered within Steam.
And a key purchased in the original Maniaplanet store does not work directly on Uplay or Steam.
Note that keys that were sold buy Steam and Uplay are indicated in batches, perhaps once a week or once a month, I don't know exactly, to Nadeo.
But for the other direction, e.g. if you want to use a regular key in Steam, the Ubisoft support needs to be contacted so they can transfer the key to Steam.
Now, once a key was registered successfully (this works automatically after installation and lauching the title in Uplay and in Steam) to a Maniaplanet account,
then the corresponding title should be available also on the standalone Maniaplanet version.
This is especially important if you intend to use the "multititle titles" on servers, because only the standalone version of Maniaplanet allows a seemless transition between the different titles.
In Steam, a title is always a separate instance of the game, this means you have to exit e.g. Canyon and then also the Maniaplanet instance, to be able to play Valley, etc.
What about Uplay, I don't know.
But I can't prove all this, since I did neither purchase keys in Steam nor in Uplay.
I only had registered existing Keys in Steam a long time ago, and I did it only for Canyon and Stadium,
but not for the other titles, since I'm only using the standalone version.
Miss, I don't know if there is really such a complicated check. I think it is only a check of the account against the keys stored in the Steam database.
If a key is not there, the game cannot be registered to Steam.
And because of the multitile issue, I think that adding external Maniaplanet titles to Steam is useless IMHO.
However I can understand that players want all their titles registered in Steam simply for easy backup.
If you buy the title/the key in Steam, AFAIK you need to install it and start it from Steam.
I don't know if the key is already available if you don't do that.
If you never touched Lagoon in Steam, you should be able to tell perhaps?