You are planning to make a 12mi trip in the same time it takes me to do 7.5 with a headwind. I thought I was slow, but... You also don't plan on riding in the rain but what if it starts to rain while you are already aroad? Personally I would strongly consider a dedicated commute rig. You can do it, cost should not be an issue, neither should space. There are bike co-ops in every city where you can get almost commute ready bikes for ~$30. Another ~$50 and you have a bike with fenders and lights, etc. and Bob's your uncle. Btw, fenders don't keep you from getting wet they keep you from getting dirty. You want them. If you don't already you should think about those suspension hoists that let you raise a bike out of the way. Or the wall mounted ones that let you mount two bikes, one above the other. You will know you are really ready for this when what I am saying starts to make some kind of sense and you look for ways to empower the desire to commute rather than try to make the idea conform with the parameters you want to enforce on it. Good luck.

H