You can not keep settings, You have to start from scratch.
Yes you can use Luci to to make the switch to Openwrt, If you want to go back to GL firmware you use luci also -- and no you cannot keep settings for that either.
One thing you need to remember is that Openwrt Uses 192.168.1.1 and not ..8.1
Not sure what you mean but if you mean the Backup and restore in luci then No they are not compatible either.
There are too many differences that if you try it will fail nicely and tell you NO this cannot be done. (Not tested so don't try)
It is possible to compare the files from both firmware's using cat or nano to give you a idea of how to configure both the same, But it is probably faster just to use luci and start over brand new it will be faster
Make yourself an escape route
-- Download a copy of gl firmware and place it on your computer somewhere.
-- Make a backup of GL firmware and place it in the same spot as the firmware.
When downloading openwrt firmware do not use the snapshot version, This does not have luci installed and must be installed using SSH
Also make sure the version you use is the newest stable version which at this time is 24.10.1
Also make sure you use the sysupgrade version