I'm in the middle of an deal for a legal two-flat in Chicago with FHA financing, but the numbers only work if I convert the basement into a livable space.

This was 1972 construction, not in Honolulu, and I noticed FLAT rent growth since 2015.

Education and knowledge will help mitigate a lot of fear and doubt on your first deal but won’t remove it altogether.

Wondering about providing washer and dryer in a two flat. 

(We're replacing the tub altogether though)

From your math it looks like you have a good range of where things need to fall except for PM costs - you should budget around 7% and it would unlikely hit that with smaller # of units such as these, I've seen PMs charge flat fees. 

Could they stay flat?

These flat fee brokerages typically abide by state law and leave it at that.

It's not a huge amount in the grand scheme, and you may end up spending more than $2,250 if she damages the unit or stops paying rent altogether...

Remember a finished basement is not included in above grade square footage but they normally assign a flat value to the finished basement.