1. You should be able to keep the CO2 in the freezer, but you will want to make sure that it remains upright; otherwise you will need a tank with a siphon in it so you aren't pulling liquid CO2. Beware/careful of condensation buildup on your gauges, as well as rust on your tank (especially if it is steel).
2. If you are dealing with low pressure, after your regulator, go for it! I can't say I've seen any home made manifolds, but it certainly is doable. I would strongly suggest the check valves as stated, as well as on/offs for your connections. MAKE SURE YOU CHECK IT FOR LEAKS! Many of us use soapy water or starsan solution to check for leaks- turn on your CO2 spray it down and check for bubbles. Would suck to come home to an empty tank.
3. Unless you have a need to replace the door, I wouldn't bother. The door has more insulation then masonite/plexiglass.
Hope this helps!
