#2
The input of an amp expects an instrument level signal, which is much smaller than your laptop signal at line level, so yes there is a risk. That said, I've run my ipod into a tube amp input and it worked fine, just make sure your gain and volume are 0 to start, then adjust slowly. Ideally you would run the line out of the laptop into the effects return but I don't think that amp has an effects loop.
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#3
^You can run a line level input into a tube amp's input. It'll just distort like crazy if your volume is too high. And it wont sound very good if its not a pedal of some sort. Most MFX units like POD's and RP's put out a line level signal. Ive seen plenty run such pedals direct into a tube amps input without any adverse results.

If youre planning to play music through your amp
a) It will sound terrible since the amp will naturally cut out a lot of low end and high end. Plus the JCA20 isnt the sort of squeaky clean amp you want for that purpose.
You want to run your laptop through a tube system, you need one of those Hi Fi tube amps, like the stereo thing that ZVex makes or the Carvin TS100, or one of the myriad tube stereo system manufacturers out there.
b) You risk damaging the speaker, since guitar speakers are not meant to put out the low end of a bass or kick drum, which im sure most of your songs are full of.