CX won't do away with seniority. Current S/O's will retain their number, obviously higher than that of a DE F/O and when the S/O is upgraded he/she will jump the DE F/O. A current S/O will still have a command slot before a DE F/O.
Don't know what pay scale they'll be on. Possibly a Freighter or an S/O scale with some sort of housing allowance. Again I'm not sure.
Doesn't need to be run by the union, the company can do it as per the current CoS, bypass pay will have to be paid though (3 months after the DoJ of the F/O). Whether the union likes it or not, the company can do it without even telling them.
Plenty of S/O's with the experience I agree, however, as per the training programme, 60 sectors for an S/O unrestricted versus 20 or 30 for a DE F/O, clears up the training program when trainers are in short supply.
Lastly, maybe the company will do that. However, S/O's and F/O's when employed by the company have different experience levels, just as an S/O is recruited with less (or different) experience than an F/O on the Cargo fleet. Freighter guys generally have (medium to heavy) jet time whereas S/O guys normally have medium turbine or piston time. Makes a difference when recruiting and as far as CX is concerned makes a difference to training.