The longer curing time the stronger the final glue joint.
As an R/C modeler, I use epoxy for various purposes, usually when strength is required. Example: Joining a pair of wings in the middle. You don’t want that joint to come adrift in mid-flight!
I won’t use 5-minute epoxy for anything. When the task requires the use of epoxy, I use at least the 30-minute variety, and prefer to use 90-minute. And for something like a wing-joint, I leave the joint for 24 hours to set up completely before disturbing it. (I press the wing-halves together with weights and leave it for a day.)
Yes, you can add alcohol. I will mix up a batch of “thinned epoxy” by adding alcohol, and use it to fuel-proof engine bays by painting it on. Properly thinned, it soaks into the woodwork before setting up. I avoid using 5-minute epoxy even for this, because 5-minute epoxy has a slightly rubbery texture, even after it’s fully cured.