Questions about Red Pro

In principle, yes, but you should consider that in a RAID 1, the weakest component always sets the clock.
When setting up a new RAID, everything goes well, but at a later point in time, there may be additional problems.
My recommendation: Take two disks that are exactly the same.

Of course you can use the WD Red Pro in the DS224+. Only the Plus models have been tested and included in the compatibility list by Synology, but the Pro models work perfectly. A RAM expansion can never hurt, whether it makes sense depends on which services are running on the NAS and what the intended use is.

Air