I've had 2 M9000s and loved them both. The first was a open station and the second has a cab. My biggest complaint is the cab is small to me but I'm big. Don't know which model you're looking to purchase so I'll mention both. The open station is very comfortable with plenty of room. Controls are easy to reach and operate with no real complaint. This may be a fluke but the cab model has more HP than the open model.
A 10.5' cutter shouldn't be a problem, I pull a 15' batwing with no issue. I use a 9' Kuhn disc mower and pull a 504 Vermeer baler, it handles all these chores with easy.
Keeping the radiator clean makes a huge difference on the A/C performance.
If you have a second remote with float chances are it will be very hard to pull it out of the float position. I took the back cover/cap off the remote valve itself and did some mild polishing where the detent balls lock it in. Simple to do and make a nice difference.
Front and rear diff locks which I have used 2x now and in both cases it was a great feature to have.
If you don't have a loader I suggest adding suit case weights, I run 400 lbs all the time. I also ordered mine with cast iron centers for the rear wheels, this adds almost 1000 lbs. Speaking of weight, you asked why Kubotas are lighter than other brands. The M9000 is a utility tractor, not a ag or row crop. Compare apples to apples and check the weight of the other colors utility line and you'll find they're closer than you think. The others could be heavier cause they're filled with BS, computers and junk you really don't need. The M9000 is a basic workhorse without all the crap. Simple and to the point without all the bells and whistles but yet enough comfort to sit on all day and still be able to walk when you're done working.
I don't know if I've covered enough to help you or not. If you would like to know anything specific just ask and I'll do my best to answer you.
I'll end with this. The M9000 is a great tractor in my opinion for the work I do. It does everything I need and from what you mentioned I see no reason it wouldn't do the same for you. I wouldn't hesitate to buy another one for myself. They are very agile with a tight turning radius with plenty of power yet surprisingly easy on fuel.
Sorry for being long winded, hope this helps.