1. Zero turn: unsafe on slope I'm told
They can be. Best to check your options.
2. Tractor: Safe? My slope is probably borderline. Perhaps 20 degrees at some points, more or less.
Tractors are rated pretty conservatively Again, best to check your options
3. Larger walk-behind: How are these on slopes?
I've always liked mid size walk behinds. They have front casters, so best to home test one- but a good candidate.
Can you add a bagger? Yes
Can my wife handle one (she actually likes mowing the lawn)?
Bigger kahunas than I. I'm pretty sure you won't see my misses dragging along behind a MSWB -with or without a sulky...

Thoughts?

While not having a serious sounding name, Gizmow makes a mower just for your type of mowing. It's a commercial duty ZTR with steering wheel control and geared front casters for unprecedented control on slopes. Below is a link with photos and videos of it working. Check it out it's quite amazing.
http://www.gizmow.com/videos-photos.htm

Here is a link that might be useful: Gizmow

I am mowing a tad more area with an old sitdown and a trimmer. If you mow often enough to mulch, the mowing goes faster. Mine takes me about an hour. I drive the mower up and down the slope. I have had the sitdown for about 7 yrs and have had no issues from running it on the slope.

Thanks all. I think a sit-down would work, but I did order one and send it back right after I bought the house. That's when I got the HRX. I know it would save time, and would help immensely in leave collection as well (I live in a neighborhood called "Oak Knoll").

I'll look at the Gizmow. Saw it some time back. Looks expensive.

On the mid-size walk behind, some of those are enormous. The one that looks more reasonable is the Billygoat Finish Mower. 32", but not a beast that looks hard to handle. Any thoughts on that one? What do those run?