L.L.
First and simplist - yes, mix your milk and the formula (and you are right, human milk or formula is better than cows milk until age 2 years). This worked for us. Start with 100% your milk in the bottle/cup for a few feedings to get her used to something other than water in the bottle. Once she is no longer surprised by that, go 75% your milk, 25% formula for a couple days, then 50/50 for a few days, then 75% formula/25% your milk for a few days, then formula.
Have you condsidered pumping a3 or 4 times per day while you are gone so you can resume nursingto sleep when you get back? If you do this, it will be easier on your kiddo. Mean while, you can get your baby used to the bottle/formula and when it is just the care taker and your baby while you are gone, your baby will deal with going to sleep with the bottle. Also, if you don't use pasifiers, I would arm your caregiver with a couple. If you are worried about it being a habit, just hide/toss them when you get back and it will just be associated with your absence, not a new part of your household.
My baby is exclusively BF and nurses to sleep (now 14 months old). She is NOWHERE close to being ready to wean, but I have had to take a couple of buisness trips in the last 6 months (3 total) in which I left her with my mom who was armed with a soft top sippy cup, my frozen milk and a pasifier (we are otherwise a no pasifier zone at this house). She said it took a while for my daughter to go to sleeep, but between feeding and then rocking with the pasifier she was able to get her to sleep and she took her normal length naps/night time sleep. We then returned to normal when I got home.
Puping for that short time was worth it for me and this worked well for my dd. I feel better knowing I am not forcing a premature weaning (before dd is ready) and I sure don't want to pay for formula if I can just keep breastfeeding.
Good luck mama - that is a tough spot to be in!!!