I'm flying to the US somewhere in the middle of August 2019, and I would like to spend a few days in Istanbul on the way/way back. As such, the website for the e-Visa application asks for proof of a return ticket. I will only have proof of an onward ticket for this. Can I still obtain an e-Visa somehow?
It is perfectly alright. All you need is a confirmed ticket out of the country. It doesn’t have to go back to the origin.
Onward in this case is as good as Return. I have done this a number of times, and have never had a problem.
Think about this: If an eVisa disallowed a transit visit to the country, Turkish airlines would have lost a ton of its customers who avail free transit stopovers in Istanbul thanks to eVisa.