Looks like you would be better storing milk in the fridge, adding until you get a bottle full and then freezing (generally recommended to add cold milk to cold milk though). I guess this would work best if you were expressing several times a day though.
Discarding unconsumed ebm is the conservative/safest option, but probably not necessary. I was told by the midwives when in hospital and exclusively expressing to use within 1 hour, but ds2 was a well, full term baby. You'd want to be more cautious with a baby with any issues.
1. When you freeze milk, it is best to use a new bottle or bag for each time you express. Otherwise the new, warmer milk partially defrosts the already frozen milk before everything is frozen again, which allows bacteria to grow.
2. Similarly, when your baby's spit has touched the milk/bottle, the general advice, to my knowledge, is that you shouldn't then re-refrigerate or use the milk again, but rather chuck it out once the feed is over.
3. You can warm milk in a bowl of warm water, or run the bottle under a hot tap.
I express in the morning every day and usually get about 5oz. I put this into a new bag and them into the fridge to cool down, then freezer. So new express new bag. When I get it ready for DD I defrost at room temp enough milk, put it in a sterilised bottle then put that in the fridge. When ready to give I warm it up in a jug of boiling water until its skin temperature. Generally DD will neck the lot so never usually any left over but if there is I'd reuse it if its been out less than a couple of hours HTH
My understanding is this: yes you can add new fresh to frozen stock as you describe. I apply a six hour rule to any bottle ds has drunk from or which has been kept at room temperature. By some accounts this is overcautious but as I only decant small amounts at a time it is not a problem. Ds will drink milk straight from the fridge - other babies might be fussier.
When I express when do I mix the milk together to get a bottle full? I've got milk freezer bags do I keep adding to the bag until full? So fresh goes on top of already frozen? Also when the milk is in the bottle I've been told it lasts for 5hrs at room temp, does that mean baby can feed from that bottle and if not finished can have the rest later? Same milk, same bottle, no sterilising. It doesn't have to be chucked like formula? Also is the milk fed to baby at room temp? Not heated? Think that's it for now, I'm sure I will be back!