B777 First Officer reported several lavatories on the aircraft became inoperative during cruise in addition to those already on MEL. Flight crew diverted; then completed flight to original destination.

Narrative

Background lav info / prior day flight in same aircraft: CA and I; along with the lead Flight Attendant (FA) on event flight (ZZZ-ZZZ1; Day 1); had all operated the prior day's flight from ZZZ1 (ZZZ1-ZZZ Day 0) using the same aircraft (777-200A #XXXX). During the prior day's flight; we operated under two lav-associated MELs.MEL XXXXT (Defect: Forward lav waste blower inop with Circuit Breaker (CB) Z-1ON P110 PNL popped; vacuum blower(s) inoperative and deactivated. Associated lavatories will not flush on the ground or in the air below 16;000' Actions - associated lavs mut remain secured closed on the ground and below 16;000'; above 16;000' advise FAs to unlock lavs; before descent below 16;000' advise FAs to lock all (associated) lavs). The following toilets inoperative below 16;000': 1R; 3LC; 4R.MEL XXXXL (Defect: One or more lavatory toilet flush system(s) inoperative and deactivated. Associated lavatory(ies) is not useable. Toilet 1R will not flush. Note we called station operations at ZZZ1 prior to departure In Accordance With (IAW) MEL to 'confirm with station control that approval for flight has been obtained from Company Communications System - operations manager.' We were told by ZZZ1 station ops that the flight was approved.During the flight from ZZZ1 to ZZZ; all toilets operated as advertised with MELs (1R was blocked off; and would not flush; 4R and 3LC were blocked off below 16;000' but were used and operated above 16;000').For the event flight; the initial release (XXXX Release 1) included both MELs (XXXXT and XXXXL). However; prior to pushback; the release was updated (XXXX Release 2) for removal of MEL XXXXT; but MEL XXXXL remained in effect. This meant there was no longer a blower restriction and only toilet 1R was inoperative and blocked off/closed because it would not flush. We called ZZZ station operations IAW MEL to 'confirm with station control that approval for flight has been obtained from Company Communications System - operations manager.' We were told by ZZZ station ops that the flight was approved and the ops manager's initials were 'Taxiway 1 2.'While in cruise after departing ZZZ; FAs called CA to report that only 2 toilets were working (one at 4R and one of the three at 3); all other toilets would not flush (with bowls full); with long passenger lines for the two operating toilets. Another call back from the FAs a couple minutes later clarified that there were three working toilets; one at 4R and two at 3 (3L and 3C; 3R was inoperative). Later (after we had talked to Maintenance Control and trouble shot the issue) the FAs called again to state only two toilets operating; 4R and 3LC.The Captain called Dispatch and had a patch with Maintenance Control to relay the issue. Maintenance Control suggested that we try to close the manual valve for one of the inoperative toilets and then try the other inoperative toilets to see if that restored operation. If operation was not restored; we were to return the manual valve to its original position; and then try moving the valve on another inoperative toilet; and again checking if operation was restored on the other toilets. The CA took over all flying duties and I went to the cabin to attempt Maintenance Control's fix. I first moved the manual valve at 1L and then checked inoperative toilet operation; starting at 3R (1R would not flush in any case and was not to be used per MEL XXXXL). No toilet function was restored. Due to the time required to move a valve (one at a time) and then check remaining toilet operation; I showed one FA how to accomplish the procedure; then had her demonstrate the procedure to me and returned to the flight deck while she trouble shot the remaining inoperative toilets. The trouble shooting resulted in no change - no toilet operation was restored and it appeared only the toilets at 3LC and 4R were working. We also learned from Maintenance Control that the operating toilets were all using the same waste tank - 'Waste tank 1.' While trouble shooting the toilets I also viewed the Lavatory/Waste tank status page and observed that the waste tank 1 was 'yellow' and indicating approximately 3/8 full. Tanks 2 and 3 were indicating completely empty.During the trouble shooting period we continued on course passing ZZZ2. CA and I talked about diversion options and considered ZZZ2 and ZZZ3. Maintenance Control advised that for the number of passengers (>300); the single waste tank might have enough capacity for us to continue to ZZZ1. However; at the same time; the FAs were expressing that the passengers were becoming very impatient and upset with the situation; that there were as many as XX people in line to use the lavs and that they were 'nearing a mutiny.' The CA and I agreed that the safest and most caring course of action was to divert. While it was possible that the single waste tank might not be the limiting factor; the long lines of passengers waiting to use the lavs was not ideal. We also agreed that continuing to ZZZ3 versus turning around to ZZZ2 was the best option (we were slightly closer to; but past ZZZ2 at the point of decision).CA told dispatch the plan; then briefed FAs; then made a PA to the passengers. We then advised ATC and received clearance to ZZZ3. We prepared for the arrival; approach; and landing and reviewed the diversion guide. The rest of the flight was uneventful. Accomplished diversion recovery guide at ZZZ3 and accomplished flight to ZZZ1 from ZZZ3.In hindsight; I'm not sure why the MEL for the inoperative blower (XXXXT) was removed between release 1 and release 2 - I question if it (blower) really was really fixed as these associated toilets (with the exception of 1R for known inop flush) were the only toilets that seemed to work.In order to secure their turn for the limited lavatory use; passengers were cueing at the toilets. While we were fortunate to have a smooth ride; had there been turbulence; this could have been a safety issue with passengers deciding using the lav was more critical than remaining seated with the seatbelt fastened. Another consideration for operating with limited lavs - but not something we foresaw on this flight as the MEL in effect (XXXXL) was only for a single inoperative toilet.