.- Is there a kind of person who could help me with pre-sale improvements (roughly saying, to tell me what is worth replacing, improving, etc and what is not,1 to make the house more attractive for the buyer)?

For example, redoing cabinets in the kitchen or replacing them completely..

I think I'll just pay some sort of consultation fee and have a engineer or architect review them and make any necessary corrections.

Under property management costs, usually just the standard 10% of monthly rent is often referenced but the lease renewal costs & tenant replacements costs are not mentioned quite as often.

Get a few quotes. and if you do not do it, when you go to refinance into a long term loan the appraiser will likely mark the report as "subject to" a structural engineer report of the foundation. and you will not be able to refi until you have done the  work that is required. 

If the roof needs to be replaced, who pays?

We have had systems fail and need replacement as soon as 10-12 years, and others lasting 30 or more years. 

If you found a replacement tenant that he found suitable and moved them in before you entered in any unpaid portion of your lease, I would expect you to receive a full refund of the security deposit.I would check with the state laws and see how much you can sue him for in small claims court.

I'd think you'd need to list both addresses as the replacement property concurrent with the single APN.

This is not just about replacing your current income.