Will already be in Germany and need a one way back to JFK on a specific July date...
price for just one way is $1600 Economy (regular) - (UA codeshare on LH);
price for same departure as above WITH a random return a couple months later is $800 Economy (regular) or $940 Economy (refundable for return).
On the second option, if I only fly the first leg and then cancel the return leg, will I get a travel credit that I can use for future domestic travel? ...
You will get a travel credit BUT it must meet staisify the the original fare rules and a domestic only leg, may cause it to reprice as a one-way. There is not easy answer and a deep read of the fare rules / routing rules are required. Additional that credit must be used within 1 year of the original purchased date, not 1 year of the FFC issuance.
... if better to book the return as Economy (regular) or Economy (refundable)?
The refundable fare will change the method of refund but it should still cause a fare recalculation.