Hi K.! Congrats on the baby.
I am a working Mom too and pumped at work for both my children and plan to do the same for our third (currently cooking - lol.)
I pumped a lot of excess milk off when I was home on maternity leave too. Then, when I went back to work, I pumped upwards of 4 times a day. I use a medala "pump n' style" pump. What I liked about the medela system was the hard palstic containers that I could freeze the milk in. They hold about 2.8 ounces and we could take out multiple containers on an as needed basis.
When at work, I made sure to have frozen the ice pack (that came with the pump) the night before and then would keep it and the fresh milk in the fridge until I left work. The milk should be just fine that way.
I always tried to defrosted the next days milk over night in the fridge, if we weren't using fresh milk from my pumping sessions. Typically, Monday was a frozen milk day for us and the rest of teh week was fresh. The milk from Friday usually got used for cereal and whatnot over the weekend. also, I was taught to never, ever microwave the actual milk. I was told to heat a cup of water (we use a 2 cup pyrex measuring cup) and set the bottle in there for a few minutes.
As a general rule of thumb, I used the "Rule of 5" - 5 hours without refridgeration, 5 days with it, 5 weeks in the fridge freezer and 5 months in the deep freezer. I agree that is best to freeze it aas soon as possible though and not wait until the 5th day.
Now, as for taking the breastmilk from a bottle, I have written about this aspect before. The biggest mistake with my daughter was exclusively BFing her. We had a huge battle of the wills over taking a bottle - of any kind! You can search through my responses to read my story but the short version is she had no choice and neither did we - we had to "wait her out" and it was one of the most painful experiences as a new mom! Ultimately though after about 14 hours she started taking a bottle - minimally at first and then hungrily. Then we had to keep her guessing about what was coming her way - a breast or a bottle. After about 2 weeks, we were able to settle into a routine and switch back and forth with ease.
My gut reaction is not to confuse the issue by introducing formula. It is sweeter than BM and for that reason often is preferred by baby. Also, it flows a lot quicker from a bottle (it is so much thinner than BM) so, if you must give the baby formula, use the lowest possible nipple size. On the other side, BM can be thicker and my little man would get frustrated at the bottle so we used the larger holed nipple for BM. If you want your little one to have BM, than that is what is on the menu and that is what he/she must be "forced" to eat - no choice. It sounds like you are caring for your milk just fine so there should be no concern about spoilage. I will tell you though that whomever is feeding the BM should be able to quickly determine if it has spoiled. It smells really sour (at least to me) and in some cases can curdle - although this is NOT the same as seperation of the milk and the fat layer. And if you are using the smaller storage containers, you can check each one individually with a quick sniff before adding it to one large bottle - this avoids having to toss a whole bottle of your liquid gold.
Best of luck. It sounds like you are doing everything right to care for the BM prior to feeding your baby. Now, it is just a matter of getting your baby on board! :-)
~C.