So...
A teammate in a 3v3 tournament has basically accused my other teammate and / or the tourney creator to force him to join the tourney,
perhaps because he didn't decline in time, or perhaps truly by no other means than he forced him to join...

And I'm looking to see if anyone else has heard of or experienced anything like this?

As in, is it even possible, is it a bug, etc. :)

The tournament itself, they must have been invited to, and they must have joined willingly. You cannot force someone to join a tournament or lots of people would have been complaining about it for years.

Once the tournament spaces are all filled, the tournament will automatically invite players to the actual games. You cannot decline these games (as far as I know, I haven't tried), because it's a tournament you accepted and joined; this is the team, if you don't want to play a game, tough, your team can't replace you now, so you have to join.

Suppose once the tournament invites you to a game lobby, you neither click join or decline. At some point, the tournament can't progress to the next round until this game ends. For this very reason I am pretty sure that I have seen a button called 'Force Join' appear by the players name in the lobby (it was a very long time ago now). I don't know how long it takes before this button appears but I always assumed it was the same as the direct boot time for the tournament game.

In conclusion, your teammate may have been forced to join that game after a fair amount of time, but he was not forced to join the tournament, and sooner or later, he would have been forced to join the game by someone, so he has little grounds for complaint. He should not have joined a tournament he cannot fully commit to.