"Internal Exception: java.io.IOException: An existing connection was forcibly closed by the remote host". I've not been able to find any discernible pattern to this problem. The only thing I notice is a few moments before the boot, the connection gets notably worse. I'll drop from full bars to maybe 1 or 2 bars then start lagging very hard.
You said it yourself, your connection gets worse then you get booted, and the boot warning is one thats typical with a bad connection to the server
your internet is the issue, speak to your ISP about it, or contact Mojang if you really think your net isn't the issue, as the only other thing would be the realms end, which is unlikely as if realms were acting up like that many people would be suffering the same