You are "TDRL" Temporary Retired Disabled List. What that means is that the board says that your disability may not permanent. TDRL is up to 5 years, where you will need to go to medical appointments. At any time in those five years that you do these appointments that you are found "Fit for Duty" or they believe that your condition is not improving, you will be taken off TDRL. If found "Fit for Duty" you can go back in. If found that your condition is permanent, the current medical evaluations will be looked at and you will be placed PDRL (where that 70% could possibly drop or remain the same) or get severance pay (Depending on time in service).
Yes, you do fall under the UCMJ. Even regular retirees fall under the UCMJ. As long as you are collecting or can collect military retiree pay, you fall under the UCMJ. Even if you took the VA money because it was more, you still could change that back to the military pay, so you fall under the UCMJ. (Not trying to beat a dead horse, but I think I nailed it!) In your case, I would think that if you went to one of your appointments and they found non prescribed drugs in your system you could possibly get busted or lose that honorable discharge and be pulled off TDRL and all pay stopped. If you miss appointments that you are scheduled for you will be dropped from TDRL and all pay would be stopped (Failure to obey a direct order).
Yes, a VA rating can go down. I haven't heard of it happening often, but it does happen. Usually because it it found out to be a fraudulent claim. Once this has been a condition that has not improved or has stayed static for 5 years, then it usually will not be lowered. Usually requires a VA re-examination.