Your taxes are none of the ex wife's business and this would seem to address your concerns.I would also put her on notice that future requests for financial discovery will be two-way and if a discrepancy is seen in the other direction you will seek support from her.

If you buy the single now your DTI will be negatively affected for future properties.

My concern is whether I’d then be locked into only larger deals in the future. 

Most STR markets have seasonality, and there are untold issues in the future that will effect cashflow. 

To establish a 50-50 ownership in both the property and future profits, you'll need a formal agreement outlining each person's contributions and rights.

My only concern to look out for is that things can go 'rural' pretty quickly if you start venturing outside of town, and most lenders are going to put the brakes on financing if the appraisal lists the property as 'rural' (has to do with what they perceive as the availability of future tenants).Watertown, NY - It probably is as good as you believe, but the lending licensing is pretty tricky up there, so its just  bit more difficult to find lenders that operate up there.

Do you know what investment strategy you plan to use in the future?

I will be posting in the future.

Generally, lenders base their refinance valuations on the property’s in-place income and existing condition, not future projections.

I am looking to purchase more rental properties in the near future.