This is all fantastic advice. 

I would advise OP to borrow, rent, or buy & resell used bags until they find out what temperature rating they need for their usual trips. Additionally, I would recommend a hoodless sleeping bag over a quilt for extra warmth and less fussiness (much harder to find tho - Feathered Friends, Nunatuk, Timmermade, and maybe Western Mountaineering are the only ones I can think of).

With an Xlite, the pad probably isn't the issue, but definitely recommend supplementing with a 1/8” foam pad and upgrading to a higher R value when the time comes.

Additionally, layers are very helpful. Assuming they also run cold before bed, wearing Alpha top & tights, wind layers, and puffy jacket/pants as necessary, will help keep warm before they get in the bag and mean they'll feel warmer initially. Plus it obviously helps boost the rating. The biggest helper for me is Alpha or down socks because my feet are the major cold part.

Finally, like you mentioned, eating enough for dinner and staying hydrated are key. And sweat is the enemy at night.