That's still a good usage of the hardware. Main thing is that it does the job.

I see no problem putting all the machines into a single cluster. By the way, what are you using for shared storage?

As mentioned, if a drive dies, you just need to take it out and replace with a new one and start RAID rebuild. The vendor should have a guide on this with detailed steps.

Well, on Windows, for bit rot prevention there is ReFS but the problem with it is that can go RAW for no reason. Happened to me several times. As to RSTe (Intel vROC), poor performance and also not reliable. Plus, not sure how the migration would go if you want to transfer to another system.

Yes, that's possible and it will protect against a drive failure. But of course, you'll get the speed of the slower drive. Also, keep in mind that RAID is not a backup.