I have a Netgear R7450 on which I have installed the latest stable release (21.02.1) of the R6800 firmware
as the web page for that device mentioned the R7450 as being a clone. Upon further investigation on the
wiki and looking at dts files in the source, it looks like the R7450 is closer to the R6700 v2 for which there is a stable release. There are snapshot releases for the R7450, but from what I can tell looking at the dts file there probably aren't any differences from the R6700 v2 images and I don't want to deal with the potential problems of a snapshot anyway.
Q0. Are the R7450 and R6700 v2 really so close that the firmwares are functionally identical except for any
strings identifying the hardware?
Q1. In general, is it safe to jump between images that are for almost equivalent hardware? What is the best way to do this while maintaining any configuration that I may have already done?
Q2. Same as Q1, but for my specific hardware/installed version if it matters.
Only someone with that particular device on the desk and after a rather thorough investigation could answer that. The generic answer would point out a rather material risk, in case flash partitioning and/ or sysupgrade specifics differ between the devices. It is never safe to flash 'the wrong' firmware on a particular device, as embedded devices like these are not generic and therefore the bespoke firmware images have to hardcode/ "assume" a lot if critical information. This might even diverge over time, with future versions - while the "wrong" image might work today, it might break fatally in a future version (as it only has the "right" hardware in mind).
It seems like you are saying that my original installation of the R6800 firmware was a dangerous mistake even though the wiki page calls my device (R7450) a clone. The way things are now that wiki page is likely to cause other people to make the same mistake. Should the wiki page be changed? Should I give up on having a stable OpenWRT installation until the next major stable release when hopefully the snaphot release for the R7450 becomes a fully supported release in it's own right.
More generally, it seems like vendors sometimes sell (virtually?) identical hardware with different model names. Will the OpenWRT project maintain multiple builds just so a firmware image exists that calls out each of those model names? (Actually looking at the wiki's table of hardware it looks like sometimes a name is listed with a pointer to a different hardware's image which would probably solve the problem of clones if it was done consistently.)
Admittedly vendors also like to sell completely different hardware under virtually the same name which is its own problem. I've purchased routers that I thought were supported by DD-WRT or OpenWRT to discover the vendor had changed the hardware without changing the name.
The images for the R7450 are snaphots and the wiki makes it clear that you can't trust snaphots for production use. And if you don't download all the packages that you might need in a 24 hour window horrible things can happen if you add packages later.
A snapshot for the correct device is still significantly safer than forcing a wrong ("stable") image onto your device. Even hardware clones might differ in the software (partitioning, sysupgrade) side.
Some packages won't install, because of dependencies, usually to the kernel.
You can always make a local dump copy of the whole package repository the same time
you install the snapshot, then you'll have all the resources you need to add new functionality.