This is a problem that is big now, but I think can also be solved with maturing the technology in the future.
Right now I have multiple accounts for multiple bubbles, but I can easily imagine some app or website that can congregate the content coming from multiple instances and choosing the appropriate account for it to post/view with.
Thus allowing one to access bubbles that have shut each other off in one central place. Unless they do it by completely blocking sign ups in which case they isolate themselves willingly and that is also good in a way to have as an option.
If I can imagine all this as a random system engineer, surely some developers with a passion for this and open source collaboration etc. can too.
I love you for explaining this so clearly, I was actually so confused about this I may or may not have misinformed someone else, oops.