a) Remote editing of programs through Blynk is NOT supported at the moment, this is mainly because the program editing interface is quite complicated and would require a lot of Blynk widgets to make it work, probably not worth it. At the moment you can start an existing program remotely through Blynk but not editing it.
b) If there is a power failure while a zone is open, Bee will not be able to turn it off because Latching valves can stay in the same state indefinitely. For that reason you can consider using a USB power bank or something that can provide backup power when there is power break. All program settings are saved in the flash memory so they will always be preserved over power break. The question really is if a program was running and power was cut off, it would not be able to stop the zone until the power comes back. Everything else, like WiFi settings, program settings, are all saved on flash memory.
c) You cannot mix latching and non-latching on the same controller. But, you can mix and match if they are on different controllers, by using the ‘Remote station’ or ‘HTTP station’ feature. This way you can have a non-latching OpenSprinkler 3.0 control a latching OpenSprinkler 3.0 or OSBee.
d) I am not sure what you mean by ‘can only use one valve at a time’ — you can open multiple zones simultaneously.