allowing less ventilation is generally a bad idea in my opinion. New fresh air contains higher levels of Co2. If you stifle the air in the room your plants will use up the available Co2 very quickly, and then stop growing until the levels rise again.
This may not be a serious issue for you, I don't know how your ventilation system works. But, unless you are supplementing your growroom with extra Co2, you may not want to limit the amount of fresh air your plants get.
I don't think your temps are too bad. Once again, like I said, move the lights close enough to keep the plants canopy in the mid to high 70's... run longer light cycles... if you need to bud, and it's still too cold at night, run a space heater in there when the lights are off. also, if you want more humidity, they make humidifiers that blow out steam, you could pick up one of those for a little bit of warm moisture.