Re: Leaving the laptop on inside the case...
When I was at college, I would close the lid (idle) and throw my laptop in my backpack during session breaks or if I had to go to the bathroom or walk to another class (5-20 minutes). It survived and didn't even get warm.
I think it all depends on how hot your system runs normally. My V2000Z (Sempron-M, integrated graphics) ran very cool even under heavy load.
I once threw a Pentium-4 (Northwood desktop) ZE5170 notebook under my blanket to try to activate the high fan speed setting (just to see how loud it would get). I eventually forgot about it for about 45 minutes. When I removed it, it was toasty, but it didn't die.
Unless your laptop is under load or running a game or something, it should be able to last 15 minutes with poor air circulation without a problem. But of course, that all depends how hot it runs normally.
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