How can I hang Christmas lights on eaves with no soffit?
After trying a few different configurations, I've decided in the interest of keeping things straight and simple, if I mount the long side of the vinyl J channel against the trim edge and drill holes through the short edge, I'll have enough space for LEDs and power injection wires and a good straight line of LEDs through each piece of J channel, and I won't have to disturb the shingles.

Added benefit: J channel comes in 12.5ft pieces and the strands of 50 ARGB LEDs are just over 12 ft so I can take the lights down easily after Christmas if I use a velcro or semi-permanent adhesive to mount them, or leave them up (semi-permanent installation) and mount them with a permanent adhesive or nails/screws through the trim.
To anyone else considering installing any kind of lights on an edge like this, you can either mount each light on a shingle clip (select lights with wide spacing so you don't need hundreds of clips) or mount the lights on something straight (pvc, vinyl/aluminum channel) and mount that to the house so you don't need to secure as many points.