You can mix EBM as long as it is the same temperature. Express into a fresh container, then chill it until it is the same temp as what you have in the fridge, then mix.

There are specific freezer bags you can get for EBM, thaw in warm water & then put it in a bottle. You can freeze in a bottle but that's one bottle you don't have available to use then...

Don't warm it in the microwave, warm in water... it doesn't really take too long.

If you were taking frozen milk out, and you needed to use it soon it might be better to thaw first (at room temerature, not in warm water).

ABA guidelines for storing breastmilk here

HTH

ETA - I used to carry a thermos with me, I used it for rinsing pump parts but you could use it for thawing & heating EBM too. Pie used to drink her EBM at room temp, although if you are breastfeeding bub might prefer warm milk