I just got back from WDW with my DS4. We used the bus (to and from Pop Century) with no problems every single day. Sometimes we'd take the bus to AoA if that bus got there first.
We had a routine down. I was in charge of the backpack and the drink holder (from the stroller). DS9 was in charge of another bag. DH was in charge of the stroller. Even if we had to stand, we had no problems.
The only time we used the inflatable bubble booster seat was actually when we got back. DH left in a Lyft to go pick up our car (which was parked near the airport) while the boys and I waited on checked luggage. However, he forgot the car keys (which were packed in one of the bags I had). So, he had to tell the Lyft driver to come back. Well, when he got back, I didn't really want to wait for him to drive all the way to our car and then drive back to the airport to pick us up. So, I suggested we blow up the inflatable booster seat. I was actually kind of glad we got to use it (because I didn't use it at all at WDW).