There were a couple of things relevant to this in today's patch notes:
* Console "save" function can be used remotely if player is added to admin list
* Autosave every 20min (instead of 30min)
So - if you're an admin, you can use the F5 save on a hosted world now. And the auto-save timer is down to 20m!
I've opened a support ticket for my immediate issue - the server crashing, going to status "unknown" and not being restartable.
We've also had problems with the server crashing once or twice per day, and it had shut down overnight while no-one was on. Should I raise a separate ticket for those issues?
I don't think there's any way to tell for sure without checking the timestamps of the world files before and after - I'm a new customer so I haven't got used to the interface enough to figure out whether we can do that here. But "F5 - save" is what's documented in the dedicated server readme and against the "F5 - info" command to initiate a manual save. If it doesn't work that's probably on Iron Gate to fix
30 minute save interval is hard-coded and happens every 30m of game uptime.
You can press F5 to open the console in-game and just type "save" to save the game manually at any time.
These are both functions built into the Valheim dedicated server, not Survival Servers hosting i.e. they will work the same regardless of if you host the game on your own PC, or host it here, or host it with any other provider.