Yes, for the v1 hardware.
Yes, if it comes from openwrt.org, it is the official OpenWrt.
No. Don't conflate the hardware versions with the firmware. And, more importantly, understand that both of the devices ship from GL-Inet running the GL-Inet fork of OpenWrt (i.e. highly customized, with a bunch of proprietary stuff in it). Installing official OpenWrt (from openwrt.org) is optional.
To understand hardware versions, I'll point you to the Archer C7 which has many versions. All of them are generally rather similar to each other because they are using the same processor/chipset family, but you'll see differences primarily in RAM and flash storage when you look at the table.
To contrast that, see the WR902AC v1 vs the v3 which both have thesame amount of RAM and flash, but use entirely different chipsets. The firmware would not be compatible in any way from one to the other.
Now, we don't know what's different in the e750v1 vs v2 that you have, so we cannot guess if it would be compatible, or even supportable at all (it could use a chipset that is not supported by OpenWrt).
Don't use LLMs for this purpose. It's not authoritative and will likely lead you to brick your unit.
This is an important point, though... this part will be your homework:
https://openwrt.org/docs/guide-developer/add.new.device