bootloader
Because the bootloader is released only as a binary blob, there's no source code to modify and add a boot menu. So there are only two ways to load a secondary Rom, 1st way is the way dual boot patcher uses which is to flash the boot.img to boot partition for the Rom you want to boot into. 2nd way is used by multirom which is kexec enabled kernel, this allows you to load a boot.img with its kernel into memory and soft boot into it, so thers no excessive flashing the boot partition.
So yes the primary kernel in the boot partition needs to be patched. All nexus 7 2012 roms use 3.1.1 kernel released with android 5.1 but each group decides which if any backports to include (i have not really looked, but that is how it's done with linux kernels until support ends) and what additions to add to their kernel for android versions above 5.1.1, like adding overclocking. You most likely could use the last official kexec kernel and depending upon what additions were used, you would not notice. But it would be better to add kexec to
source code of your particle group (ie omnirom, lineage, etc).
Since my main rom is still 5.1.1, i still use Tasssadar 5.1.1 kexec kernel. i'm skipping 6 and currently trying lineage based 7.1.2 roms. Right now I have twrp 5.1 minimum env so can use Tasssadar grouper device and kernel for 5.0, also i
an am trying to set up lineage 7.1.2 build env with multirom added to grouper device so i would be able to make lineage 7.1.2 my main Rom.