As such, your portfolio of rental homes would not be considered a business capable of sponsoring a Solo 401(k) plan, nor would it make sense to attempt to re-configure a rental business to accomplish this goal.  

I'm set to begin a pretty extensive remodel on my primary residence where we'll be adding a bedroom, bathroom, and re-configuring the kitchen.

And that configuration can hardly be considered a house hack as I'd for sure pay at least what I pay in rent ($3600) as my portion, and likely much more. 

The office is configured as 10 individual private offices with a co-working space consisting of 4 desks.

A lot will depend on the configuration of the lot (no pun intended....ok maybe a little bit).Also look into the zoning and building restrictions in your area.

There's probably a configuration where this would make the most sense, especially if you were to be really careful in your design and construction costs.I'm definitely gonna investigate this a bit more!

It might be easier to find the right single family home and configure it for a comfortable housemate situation. 

You may be limited slightly by who would accept it, but I have never seen a drop in demand between my 1st floor and 2nd/3rd floor units that are all same 2 bed 1 bath configuration with same amenities and size.

Reason your finding so much in generalities is because Real Estate Investing is a very generalized term and concept, under which there is dozens upon dozens of main strategy forms of REI, and each of those have dozens upon dozens of implementations and configurations.

With 2400sf of space, you've got plenty of room to work with.However...Fortunately, you don't have to guess here: Recent comparable sales will tell you exactly what configurations have sold and for how much.