Your wife probably does not have a license or insurance or the expertise to supervise a renovation.

If the furnace requires replacement, who collects bids and supervises the project?

It sounds like this Ella is illegally operating as a contractor if you were paying her and she was supervising the work. 

Then you have to supervise them, at least until they've proven themselves stable.

Seems like 2 hrs is a lot...unless they spent that much time at the property supervising.

One of the sprinkler heads are broken by the lawn mower(which tenant was suppose to supervise).

It doesn't even look like that much pipe and likely something a technician would do with the plumber supervising and checking the work.

But at the same time if the tenant wanted to repair/modify the unit by his/herself I would like to supervise and make sure nothing goes wrong.So realistically, how do you handle tenants who constantly report breakdown and request repair?

If you're concern was from a marketing standpoint, being built as a low/mod income home, that doesn't need to be mentioned and, I'd put most any HfH stick built home up against any other in that price range as to quality of materials and workmanship, each building crew is closely supervised by those with experience, so it should be a sound build. :)

I supervised every part of the process and did much of the work myself.