The liquid yeast will be fine in the fridge. Yes, you should bring it to room temperature before pitching.
You might consider making a starter with your liquid yeast - there are threads about it here, if you do a search. Basically, you make a little miniature wort (boil some water and extract), cool, add the yeast, and let it work for 24 - 48 hours. Then you pitch this version of the yeast, which is healthier and more robust.
I've never used molasses so can't say from direct experience. I have added sugar or Belgian candy sugar to beers - at the 15 minute mark. One of the effects of this is to dry out the beer. I don't know the science about the fermentability of molasses, though. I say that experimentation is always a good thing!
Because there is so much variety in liquid yeast (compared to dry yeast), I'm curious about what kind of yeast you are using for your porter. English Ale, London Ale, or ? The differences between yeast can make a big difference in your beer - I find it is often the biggest factor.
Cheers!
Jim
