So in an ideal world, presuming you are cash flow positive on your investment properties, there would likely be no negative consequences.  

Not much multi family inventory on the market and if you don’t have trade skills or subcontractors it’s going to be tough to buy a good deal, hire out the work to a contractor, and still make a decent return if any at all.

When he goes to college/trade school he mentioned wanting to live in one of those apts with a roommate, possibly as an on-site mgr. 

Generally, mixed use expenses for a trade or business are prorated based on the business use.85-90% business use year-round for your only laptop seems high if you only own one property.Some things you're considering "trade or business" related might not be. 

They paid me $440 per month for 3 years and then we have been trading services ( he landscapes our properties).

Meaning that you might have issues trading up at a later date.

If I fire K. immediately and she won't e-mail me the lease agreement, what are consequences for me?

Usually this needs a promissory note and actual cash payments to be respected (as opposed to a month liability accrual).Your tax professional is best equipped to explain the specific ramifications to you and long-term consequences based on your individual facts, circumstances and goals.

To qualify for a 1031 exchange, the new property that you purchase must be "like-kind" to the property being sold.In general, real estate held for investment or use in a trade or business is considered "like-kind" to other real estate, regardless of the type of property (e.g. single-family home, multi-family home, commercial property, etc.) or the manner in which it is used.

I personally like trading up, but if you are confident in the continued appreciation that could provide a greater return with far less effort.John