I did the same, started with Lemmy World but quickly realized it was getting too big and slow.

Currently no, but there is a bit of a workaround.

You can use Memmy with Lemmy, and then you can use Lemmy to subscribe to communities/magazines on Kbin.

That’s what I do currently and it works very well.

Currently I don’t believe that’s possible.

If you want a way to view Mastodon, I’d suggest either the Ice Cubes app or Elk (website).