She may be fine with it but you have to bring it to the table and we don't know your mom so you asking tells me you suspect an issue. 

If the appraisal doesn't come in like it did 6 months ago, I won' t able to re-fi unless I can find a lender that will do something better than 75% of the value of the home. 

For loans that size, you are likely working with a bank rather than agency or bridge debt. 

My fiance and I live in a mid-sized city, where the average single-family home will run around 300-400k and could rent upwards of 1500-2000, depending on size and neighborhood.

If your rents are $1,900 for a similar size unit, you may now have some competition.

For a multifamily only your % of personal use of the building will qualify for the 121 exclusion (2/5 year rule)So if you occupy 1 unit of a 4 plex (assuming all 4 units same size) 25% of gain can be excludedYou can however combine this with a 1031 exchange and defer the gain on the other 3 units into a new rental property 

In my previous life I owned medium sized businesses - very successful.  

The important things to consider are finding a single family on a multifamily zoned lot, number of parking spaces, number unrelated parties allowed in that city/county, and overall lot size (for future development).  

Odds are good the lazy owner will never do an eviction, so that needs to come off the table.

If you buy in a location where prices and rents are increasing below the inflation rate, the amount of goods you can buy will decline over time.Population Size - Greater than 1 million.