Never heard of an ED visa requiring booked accomodations for 90 days. I'd just book an airbnb for a few days and edit the confirmation page to adjust your stay. Make absolutely no sense to book a place for 3 months in advance that you've never seen before.
Anytime a foreigner stays somewhere, hotels, guest houses, ext need to submit a TM30 notifying immigration. You need the TM30 receipt for your extension. If you're staying in a hotel they'll have done it online, just take a picture of it or ask for a screen shot.
Covid extension is like a tourist visa, you can extend it anywhere in the country. You'll need the TM30 of where you're staying though for whatever immigration office you go to.