Or, you can build up your 401k, then when you switch to another job or retire, you can always rollover that into an IRA or even a Self Directed IRA that allows investing in real estate and other assets.

So if you have to cash out or rollover, you could rollover to a SDIRA and use your retirement funds to purchase real estate. 

I don't want to make an enemy of this man and was very respectful and polite in my request, but I'm not going to just roll over and make an unnecessary improvement.

If you have a traditional roth 401k, you may need to look into setting up a self directed account and rollover the fund if possible.If you want to take a loan, keep in mind that an IRA doesn't allow that.

They also have options to roll over to more conventional financing upon project completion if that should be necessary.

You can roll over your acquisition fee in the deal but you should still have at least 5-10% of the equity coming from you.

@Joshua Massariyou didn't mention anything in your original post about 60-days rollover and I can't read your mind so that is why I answered based on what I read. 

Rollovers and transfers of existing retirement funds do not count toward the contribution limit.

Let's say my spouse sets up/funds my spousal IRA (traditional pre-tax), wouldn't I be able to direct rollover that spousal IRA to the Solo 401k (also, pre-tax and assuming the plan documents allow for direct rollovers)?

Yes you can use your contributions and even roll over amounts to add to pay down the loan.That should cover your questions simply.