Coverage is not as good as maxis. And unifi mobile separate 3g and 4g quota. If you r out of 4g coverage and u have no 3g quota, your internet connection will be dropped.

When u buy the data. The 3g quota is included. No worry. Just ensure your area is 4g covered. Then other shd be fine.

I think shd be fine.

All purchases on unifi@mobile is through the app.

Not necessarily need to topup RM10 every 90 days. Every usage will extend that 90 days validity.
With a RM10 topup for Calls/SMS, assuming 1 sms is sent every 90 days i.e  4 sms/year .. it will take 15 years to finish/expire.

You can only apply thru mobile@unifi app for the SIM and can buy the topup / add-on thru the app too.

topup can be purchased as pin from any e-pay retailers/online. but still need app to key in the pin

For convenience's sake, yes each smartphone each app.
If using someone else's phone & app, it's a hassle to logout / login for each account to do transaction.