Oh that poor little plum tree, being pulled in two different directions! lol Just tie the plumb tree to the other big tree! ;-)
Yes, I think you can still use the 7/64. At half the length, you should get half the stretch and sag. Plus, if you keep the strap up high, you can increase your angle which will further reduce tension.
7/64 has 1600 lbs average breaking strength, 1400 minimum. I'm actually using 1/16" Lash-It rated for 400 lbs, with 15' slings on both ends (just in case I found myself in the woods with the two closest trees 30' apart!) But I haven't put them to that 30' test yet, and I'm only one person. Happy Hanging!