1) the instruction in post #1 bless OpenCore in recovery partition, therefore, no need to change SIP status.
My approach is to keep it as simple as possible, but the users themselves need to know where those OC files stored. Therefore, my instructions ask the users to mount the EFI, copy the files, and run the bless tool (which required SID disabled)
2) If you want vanilla OpenCore, better go for the approach in post #1 here. Mix use different methods usually require better knowledge than average.
Of course, if you 100% sure your own config is correct, then you can download my package, remove all unnecessary kext / tool / etc, change the config.plist to your own config, then use my way to install OpenCore.
3) After you build your own config, you should know the answer. The short answer is yes, just a matter of where we put this argument in.