It's about always having enough money to maintain your lifestyle throughout your lifetime.

I would conduct an inspection, determine the cause, and make the repair.You shouldn't keep tenants who habitually pay late, but there are various factors to think about and I can't go into all of them.

The reality is that the tax impact and ability to take ANY losses (as Michael mentioned) is based on a number of factors that include whether or not it is a "separate dwelling unit".

We are in agreement that my moving into the apartment is a huge contribution but don't know how to factor it into the deal.

They can help you navigate local tax obligations based on your income sources and property ownership.While taxes are definitely a factor, don’t let them be the only one.

Another factor to consider is that if you want things to get done, put it on your end; if you don't care one way or another, put it on the tenant as long as they don't mind doing it.

Your ROI off a 20% down payment will be around 60-80% when you factor in principal pay down on your mortgage (which your tenant is paying off), 5% normal appreciation and monthly cash flow.

I've included an example below to help illustrate this.So different lenders have different rates (which do vary even for DSCR loans) but these are factors they all consider.See example below:DSCR < 1Principal + Interest = $1,700Taxes = $350, Insurance = $100, Association Dues = $50Total PITIA = $2200Rent = $2000DSCR = Rent/PITIA = 2000/2200 = 0.91Since the DSCR is 0.91, we know the expenses are greater than the income of the property.DSCR >1Principal + Interest = $1,500Taxes = $250, Insurance = $100, Association Dues = $25Total PITIA = $1875 Rent = $2300DSCR = Rent/PITIA = 2300/1875 = 1.23If a purchase, you also generally need reserves / savings to show you have 3-6 month payments of PITIA (principal / interest (mortgage payment), property taxes and insurance and HOA (if applicable).

However, I do see most rentals do 1x rent for a security deposit, so it would be another potential factor in a renter's mind when comparing your listing to another. 

If they confirm that it's an isolated event, I'd also recommend enabling multi-factor authentication on your account moving forward.