Yes, an adult mouse sounds fine. (of course, the ones I see are tiny lol)

If you feel more comfortable feeding her in the cage, it's a good bet she will too. Snakes don't associate us as food with regular non feeding handling. Go ahead and try in the enclosure.

For suggestions on a smooth transition, prescent the cage for a few hours before you feed her. Use dirty bedding, or thaw the mouse in a bowl on top or next to her cage.

When it's time to feed, make sure the mouses head is really warm. So she can lock onto it well. You can put it up to a bulb for 30 seconds, that usually does the trick.

Hope everything goes smoothly! Good luck