How long is it acceptable for the managing editor (= you) to wait after the first unanswered message to the editor before replacing the editor?
If the lack of contact is unexpected and unannounced, then wait three weeks with weekly email reminders.
If it is expected or announced, then negotiate with the authors. The authors might prefer to wait out an ...
Waiting for them seems like a lost cause. Unless you have signed some release or given them copyright, the paper is yours. You can submit it as you like.
But submitting it minutes after notifying them of a withdrawal might be hasty. A few days wait is suggested. But they don't seem to have a very good record of corresponding with authors.
I am an associate editor for two journals, and although the situation is fairly new, I expect to have no substantive change in the time for me to handle papers.
The reasons for this are:
Pretty much everything is handled purely electronically anyway, so there's no physical interaction to be disrupted.
Reviewing is typically a sparse activity by people who ...
How should I modify these in lights of the ongoing COVID-19 crisis?
I don't think you should modify anything about your approach. That is the journal and editor's job, not yours. Watch for official updates, if any, but otherwise proceed the same as before.
I am only interested in answers that go beyond such obvious thoughts
This is an unprecedented ...