How long can the smoked brisket stay in holding

I would think that you'd end up baking the item, not smoking it. You need to get enough heat to the wood chips to get them to smoulder, without directly heating the item to be smoked.

All of the times I've seen people on cooking shows attempt to rig up a smoker like you descibe, they've done it stove top, not in the oven, and it tended to be in a pan a little heavier duty than a disposable aluminum tray -- it looked more like a deep hotel pan to me, and I don't think they vented it 'til they were done.

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That being said, I still occassionally grill outside in the winter. Okay, I admit, it's usually because the power's out, but I did it last weekend as a favor to my neighbor so he could serve steaks to his wife and mother-in-law for the day before Valentine's day ... in that case, it was actually fairly warm (maybe 45-50°F), but I've also done it when we've been buried in snow and ice.

The important thing about winter grilling is to keep the lid down, and not check on things too often. For long and slow cooking (I think I was trying to finish a stew), I've rigged up things to insulate the grill ... although, I managed to cook one of my leather welding aprons last year.

But for cold smoking ... you're all set ... you can get get insulation blankets for your smoker, and then you just need to keep the fire going in the hot box.

Chewy means undercooked. Most of your standard "barbecue cuts" of meat contain a lot of connective tissue. This must be rendered to achieve tenderness. This goes for brisket, pork butt, and ribs, to name a few. If you are using the words "chewy" or "tough" to describe the texture of your meat, in nearly all cases it has not been cooked enough. Your time/temperature pretty much confirms it.

The best thing you can do is deemphasize time and temperature as your measure of "done." Rather, use a skewer to probe your meat when you think you are getting close. It should slide in and out of the brisket with VERY little resistance. Some people like to wrap in foil a few hours in. This will finish the brisket faster, as you will mitigate the evaporative cooling effect that causes your cooking process to stall. Plus, you can add some liquids and spices to the foil, which adds to the flavor profile of your meat. This is better known as the "Texas Crutch," and there is no shame in using it. Plenty of competitions have been won employing these tactics.

Brisket is a little intimidating because of the small window between "too chewy" and "dry and falling apart." But you should probably err more on the side of the latter, as that can be more easily compensated. The real takeaway here is have patience, and look to the meat itself for clues as to whether or not it is done, rather than using a timer and thermometer.