Not entirely sure what your use case is for Discord gaming on android? Mumble is the libre voice chat to beat - the android client is called Mumla. For Discord chat overall, matrix.org is your best bet, two popular android clients being Element and Fluffychat, though at the moment its conferencing is just embedded Jitsi which is not really comparable to Mumble/Discord.
Install matrix server. Although, the recommended client is Element, but I use FluffyChat because it provides voice messaging like WhatsApp along with usual chat features.
Nem is kell, a matrix.org hosztolja neked. Jelenleg kliensnek én a FluffyChatet szoktam ajánlani átlag usernek, mivel egyszerűbb mint Elements kliens.
ui.: Email szervert is hosztolhatsz magadnak, ettől még az átlag ember is tudja használni.
Hackshewally, for reliable privacy, a Matrix app with a dedicated home server and end-to-end encryption is probably best.
"Government by fluffychat" has a much nicer ring to it.
I’m not sure of an answer to either one of your questions, I’d recommend posting an issue on synapse github for both of them.
For the second issue perhaps try opening the user that can’t open messages in another matrix app like fluffychat.