You're asking good questions!
You're right that if you were to epilate a hair at beginning of telogen you might have a long wait, as much as four months, to see whether the follicles regrow. If you epilate in anagen, you have a good chance of seeing the hair regrow soon if it wasn't properly treated.
These particular hairs were an ideal marker because there never were more than one or two there. The time a hair spends dormant, with no hair in the follicle, depends entirely on the amount of abrasion the hair receives. If no abrasion, the hair will remain until pushed out by a new anagen hair. In this case, at the navel, there was little abrasion, and since no more than one or two hairs were ever observed, I conclude there were two live follicles.
Since we caught them in anagen, and I haven't seen regrowth over four months, I feel confident they have been epilated properly. Note that these hairs should have about a six month cycle, so I won't be certain until several months from now.
Under any event, the fewer hairs you're dealing with, the easier it will be to ensure that their treatment is successful.
They're sneaky little bastards, that's for sure.