Scott

I had the same issue with gravel and whether to put mulch on, which is a pain to remove in spring.
H didn't want mulch all over "his" gravel so I removed it again.
PITA, as Jack says.

Your "lots of mulch" around base doesn't look like a lot.
Hardy Jim uses, I think, 5-6 inches deep mulch.

I don't know what your min temps are, but maybe add another string of lights in case 25 aren't enough wattage.
You can always leave 2nd string unplugged, but it'd be a pain to add another string in the middle of winter.

Vent hole, IMO, might be more than you need.
A half inch space...on only one side, will let enough accumulated heat out.
And you can always tape a cardboard "shutter" over the south window to regulate heat accumulation on too warm/sunny days.

I have no experience at all with heat under/around roots, but BILLMA did that with his palms and bananas.
Forgot who else.
Seems to me warm roots/soil would need watering.

Others here will have good advice/options.

Next year you'll need something larger...

Fun projects!
Barb

Had thought of the heat tape as a common sense thing to do, and then read the "Palms won't grow here" book, and he suggested it as well.

I had read another member had done the heat tape, and never watered all winter and basically "cooked" the palm to death, so had planned on using a extra long piece of PVC tube that would end at the base of the plant and then stick out through the side so I could pour some water down to the very base without getting anything else all wet so I could still add water. So much depends on what kind of winter we have.