Break the job into pieces and pay them after each is completed.

The "home owner" is more like a tenant, in a binding contract to pay a rent for 30 years or cash to own that property, regardless of what is written on a piece of paper called Deed. 

Remember: Insurance companies want you to pay the bill and when it comes to their turn, will do everything they can to NOT pay.I would get clear on that piece before proceeding further.

No unit can be rented more than 2 consecutive years and so on.Another piece to consider is even if the bylaws today allow rentals... this can be changed at any point in the future.

I am currently in my early 20's, have a well paying W2, and would like to begin investing immediately.

You just have to learn the process and follow some  broad pieces of advice such as - don’t invest a lot in individual stocks because you are subject to much higher variances in performance.  

HOWEVER, Relay is only the front end/ tech piece.

Gues this is legal if they are sending monthly statements that an owner (or their tax professional) can piece together.Regarding mistakes, not aware of any company in the world that is perfect and doesn't make mistakes.

Another job (which doesn't seem DIY) estimate he's provided me is bringing out a stucco wall - this home is 100 years old and the wall is made of this:Outer most later stucco type of cement, wire mesh, plywood, 3 inch beam every 16 inches, thin wood pieces rubs horizontal with 1/5 gap and then stucco inside (this stucco grade is different from outside)The wall is just after the entry door to the right and if I open it up, I'll have a larger space to place a shoe cabinet and stool.

Ultimately the value of the single family home lot is the critical piece to the equation.With respect to financing the new build, assuming you receive the zoning relief and can subdivide the parcel and the parcel is released from the current mortgage you should be able to use the land towards your equity.