Here's the parts diagram for the BX2230 PTO engagement lever. There are a couple of notches in the slot on the fender - one for the "engage" and one for "disengage" position. If the lever physically won't slide far enough to catch the "disengage" notch, either the linkage is bent, fouled with crud or just seized up due to lack of maintenance (kept clean and lubricated). It's also possible something is wrong inside the HST... you can hope it's the linkage, of course.
Regarding the mower deck not lowering all the way. Don't really think low HST fluid would have much to do with this since the lowering action is by gravity; 3 point hydraulics do not actually push down.
The deck adjustment knob already mentioned is the obvious place to start. Beyond that, raising and lowering the deck is done by the 3 point hydraulics, so you should see the lower 3 point arms going up and down as you raise and lower the deck. When the arms are fully lowered, they should drop down to about 4-6" from the ground. If they are doing that, then whatever is keeping the deck from lowering is in the deck linkages, not in the 3 point system itself.
There is also a 3 point lowering speed adjustment, also near the floor between your legs. If it is all the way in, the 3 point mechanism, and the deck, won't lower at all; the more you loosen it, the faster everything lowers to the ground.
A good cleaning and lubing of all the linkages is a good place to start. Once it's clean, you'll be able to see more of what is happening.