Even if the original issue had anything to do with ISOs, he’s way overestimating the level of protection on many install ISOs, in my experience—they’re just disk images, and presumably all the files read off them are passing through Ventoy itself. Even if you find one that performs some kind of verification, easy enough to change a jump instruction to a no-op somewhere in the machine code guts of a file as it passes through Ventoy, and prevent the verification from executing. (The difficult part is figuring out which instruction to change, but people have done it before.)
The GPU doesn’t care about the CPU, or vice-versa. AMD is probably better value for money right now if you’re intending to replace both CPU and mobo, but Intel will work.
The reason you don’t see AMD CPU + nVidia GPU in premade machines these days has to do with corporate contracts, not interoperability. Before AMD bought out ATI, it wasn’t an uncommon combination.