1) Good question. Since you have a backwater valve, you don't want the cleanout to leak during a system backup. So I can see the argument that cleanout#1 should be above the elevation of the street manhole. On the other hand, if the threaded plug is properly tightened with dope, it would withstand 10 feet of head, meaning it would be fine.
I'm not sure about this one, whether to raise the cleanout or just be sure it's properly tightened. The relevant rules are here, I scanned them but didn't see anything directly addressing this:
https://up.codes/viewer/washington/upc-2018/chapter/7/sanitary-drainage#710.0
2) You have (3) WCs. So your building drain could be 3" (maybe it's bigger). That means the area of vents through the roof needs to be at least the area of a 3" pipe. That could be (1) 3" pipe, or (2) 2" pipes plus (1) 1.5" pipe, or (4) 1.5" pipes. Except WCs always need a 2" vent, so you will have at least one 2" vent, so (4) 1.5" pipes is not a possibility.
Vent #4 has to be 2", as it is wet venting the WC. Also, vent #2 can be 1.5", as it is only venting that lav. Not shown on the left are Vent #5, 2" for the shower and WC, and Vent #6, 1.5" for the kitchen sink.
You can combine the vents in the attic in whatever way you like that complies with the aggregate vent area rule. For example 1+2 as a 2" roof penetration, same for 3+4 and for 5+6. Or you could bring them all together and have a single 3" roof penetration.
3) That horizontal vent section is only allowed if the inspector judges that "structural conditions" preclude a fully vertical vent. In that case, the vent takeoff would be best done as a combo rolled 45 degrees off vertical, plus a 45 degree bend; or it could be a wye rolled 45 degrees off vertical, plus a 60 degree bend.
But is there no wall nearby between the shower and the WC? The best solution is just to route the shower trap arm under/next to a wall to avoid a horizontal vent section below the floor.
Cheers, Wayne