CO detectors will only detect carbon monoxide, unless you have a dual purpose detector that also detects propane/natural gas (Nighthawk sells them, and I strongly recommend them to every lampworker).
The reason single one pound tanks are permissible is that the volume of gas contained in the tank is so small, that even if there was a full blown leak, the average sized room is large enough that the explosive level of propane won't be met. This is not the case with 20# BBQ tanks though.
Your comment about one window concerns me though about ventilation - you will need more than that for good ventilation, unless you are venting out through a duct. The window is going to be needed for bringing in fresh replacement air.
I've never seen anyone welding in a basement, but plumbers do it all the time - and they don't turn off any pilot lights. Once the fuel gas has been burned, it cannot re-ignite - and the possibility of a leak is very small.
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