After reading the de mimimis portion, it sounds like this is just a measure in order to simplify book keeping for small-time landlords/business owners, do you agree?  

For example, if my accountant accepts my bookkeeping for my return with expenses of $2500 or less listed as expenses, is it his or her responsibility to verify and "sign off" as it were with the IRS that we have a written capitalization policy?

@Joe Black, I personally use Widget CPA & Bookkeeping in Clearwater.

This adds the complexity of book keeping and the expense of the tax return that would not be necessary with an all cash purchase by the IRA.  

bookkeeping etc.

I have no personal experience with this nor do I know offhand which banks, but you might ask around.Due on sale clause aside, you are going to pay $800/yr CA state franchise tax, plus bookkeeping fees, and extra tax prep fees?

Also, when the balance in the solo 401k exceeds 250k - does it simply mean filing the appropriate tax return for book-keeping (with no tax consequence) or does it actually mean paying more taxes?

I mainly want to keep bookkeeping and bank accounts separate and organized, but don't want to trigger any additional tax consequences.

Robust bookkeeping/reporting system - this requires a Bookkeeper and probably some Admins.

More of a book keeper but also CPA.