do you have a sleep routine or sleep association items? until recently DD was 100% bf to sleep - but we knew we needed to change that so we used sleep association items and would then get DH to get her to sleep. he now does the night time bed preparation (bum change, jim jams, sleeping bag etc), i feed her, then either one of us will cuddle her to sleep. she knows she only goes in the sleeping bag for naps, she knows dummy means nap time etc. hopefully you'll have some sleep association things that will trigger for her that it's nap time. she may struggle a little with the notion to start with, but i was surprised by my DD. the first nap she had for daddy, she was tired and just went to sleep for him - i was upstairs helping a friend do something, came back and she was out cold. sometimes i think mummy not being there makes settling for daddy easier....

i would go with cows milk in a sippy cup if she needs a drink. it can be warmed a little as a comfort thing if she needs it - you may find she is happy to just snuggle daddy and sleep without the milk feed if you're not there though. i wouldn't give her formula at this point - it would be an unfamliar taste and totally unnecessary when you're only missing one feed. just be prepared taht she might be a bit of a booby monster when you get home to make up for it!

ETA - i wouldn't NOT feed her when you get home just because it's not a normal feed time - you said you're a little worried about your milk levels - you need to get that stimulation to keep producing.