Assist feeding should only be done when EVERYTHING as be done and obviously it has not live is still an option, not sure what pet store as told you it was illegal to sell live rodents as feeder but it is not in the US (no where I can think of at least), now it might be against company policy but this does not mean it is illegal.

If your chain pet store will not sell you live mice (that is what I would go with) try finding someone locally that will that will.

You really do not want to assist an animal that knows how to eat, what you want to do is assess your husbandry which is often the issue (temps, enclosure size, hides etc) and offer live until the animal eats with consistency.