Kenmore 665.15522 won't drain?

We have a Kenmore dishwasher Model 665.15522000 which wouldn’t fully drain. Not sure how old it is – it came with the house and worked just fine for the past 3 years. Recently it started making weird noise in the middle/second half of the cycle – sounded like the motor or pump is obstructed by something. Then this week we found it filled with about an inch of water in the bottom after the cycle was complete. I reached around the sprayer/Ultra wash assembly and found 2 forks, which apparently fell through the bottom rack and ended up there. One was in plain view, but the other one was kind of hidden below the rim of the sprayer/Ultra wash assembly and stuck in the bottom of it.

I removed both forks and reached around the bottom to see if there is anything else stuck in there, but it was all clear. Then we removed most of the extra water and I tried accessing the filter but couldn’t get to it. I removed the lower sprayer arm and the “Ultra wash soil remover” cover (held in place by 8 screws) but under there I found a black rotating circular part of some sort, which seems to be an impeller of the pump or something like that. It’s held in place by 1 hex bolt right at the axis, but when I tried to remove it, it would just rotate together with the black plastic part (impeller) and all my attempts to hold it strongly enough to unscrew the bolt didn’t work.

I did however disconnect the drain hose from the garbage disposal and cleaned out all the nasty stuff that was there, even though it didn’t seem to be enough to seriously obstruct the flow. I couldn’t get to the other end of the drain hose (the one that attaches to the dishwasher itself) because it’s fully covered by adjacent cabinet’s wall.

Tried running it again with some vinegar in it after cleaning out the hose connection and it filled up with water again to about the same level. It sounds more normal now and the whole cycle seems to be running fine, but the bottom stays filled with water.

Basically, I have 2 questions.
- How can I get to the filter in this model?
- What else can I do/check to fix the problem myself?

Thanks in advance!