it's a minefield! When the check in agent picks up the phone to ask "can he travel on these docs", I'm the one at the other end making that decision. Right now, it's changing all the time. When I've been off a few days, I spend half my first shift back asking the others "what's changed?". I used to know most countries regulations by heart - since March, I've been constantly playing catch-up.
I work in aviation. The general rule applies to scheduled departure time and is (usually) flexible in the case of a delay that is outside your control (so long as that delay is within a reasonable time - the destination would normally be contacted to confirm this)
You can't "renew" a visa in this way. I think you mean "apply for an extension of stay". Depending on your local immigration office, either 30 or 45 days before your current visa (or extension) ends.