It may sound nice to pay a 6% management fee but the extra fees can add up to be more than the other company that charges 10% with no additional fees.

Quote from @Sami Gren: I start by making the original tenant fill out a form requesting to add a new renter, and they pay me a $50 processing fee (I charge a $50 fee any time I make changes to an existing lease). 

This is what blows my mind about the cults of seller finance - they teach you to pay anything for a home because you can put little money into it.

If I pay down $50K, how can I calculate the new monthly payment?

You would need to let them know you are 2nd lender and want to pay the loan off.

I am happy to pay a competent firm to keep me from violating the terms of my plan and answer questions, but somehow I fear that no one will want my account because the banking part is outsourced to Titan Bank (which is not paying interest, so I should probably switch from them too) and perhaps there are no custodial fees to be had the way this plan was set up. 

If you discovered the issue before paying for the appraisal that could hopefully save some costs (in case the appraiser is with the wrong AMC).Pre-vetting with the lender on rental history/AIRbnb/1007 as well as seasoning could help. 

So really need to do some deep market research, haha, but not so willing to pay for this, at least not at this stage when I'm just starting out.

(seems straight forward)B) Landlord denies the request for ESA exemption of pet fee / pet rent, but offers the applicant the unit if they ARE willing to pay fee / rent for the animal as a pet.Or... 

No fees, no pre-pay.