My daycare lady had a Jetta, not sure what year, but it was older. We fit 2 Maestro outboard and a Scenera in the middle.
Given your 2yo is going to need a seat for awhile and you are always going to have a 3 across. I'd buy the 45# RF one. I'd put it in the middle RF. Then, I'd try 2 Harmony boosters for the oldest two.
When I had all 4, I would put the 9yo in a booster in the front in one Harmony. 7yo in back in Harmony. 5yo in back in High Back TurboBooster if it fits? Should if the Maestro fit w/ a Scenera in there, right? 2yo RF in the Radian.
If I had the 3 kids. I'd put the 9yo and 7yo in the Harmony in the back. The 5yo in the RF RN in the middle. My oldest RF in the RN until he was almost 6yo. He still fit, but YDS needed it. Rode in it a few times after, still fit...Obviously that would be if the 5yo fits.
If the 5yo did not fit, I'd tell the 9yo and 7yo to suck up the shoulder space it is going to cost them and put it FF. Then, I'd either switch it when the 2yo rides. Or if you have no airbag, put it RF in the front. So you don't have to switch it all the time. BUT, the straps have to be coming from above the child's shoulders for forward facing. It works for my 3yo and 6yo to share a seat FF, but one is tall and one is short...
tam