I can only imagine how you are feeling. Are you really expected to attend?? Seems like it would be a nice gesture, but surely not an expectation - a friend's MIL??
Can you be honest with your friend? Just tell her you're really not feeling great at the moment, if she's a friend she will understand. With all due respect, it's not like you were close to the lady who died. Please don't beat yourself up, crikey, if we went to every friend's relative's funeral, when would we have time for a life?? Different if you knew her well and were her friend.
I have never been on jury duty. I can understand how you're feeling, it would be interesting but daunting to - that old unknown!!!
Just remember what you're therapist taught you - don't anticipate - you can not possibly know what might happen, and surely it is unlikely to be bad. Go through the scenarios that you are worried about - and ask yourself "so what" - you don't know any of these ppl - you will never see them again -just go with the flow.
You might even find it's a really interesting and fascinating experience - just remember to breath, nice and slow. And when you catch yourself anticipating - tell yourself to stop, and then ask, well if it does, so what.
I know you will be fine.