#1
So I am not 100% convinced I can name the problem, but all my experiments lead me to conclude that my input connection is lose in the head of my amp.

I have a Marshall 100MG and while it's not my favorite amp in the world, it's all I got at the moment. Is there a simple way to dissemble and fix it, or should I take it to the shop?

I am not a very good mechanical steward, so my confidence is low.

Suggestions?

EDIT: My band has 5 shows this month, including one this Friday, so I'm inclined to hurry. lol
#2
I'm reluctant to help anyone fix an MG, but assuming it's an input jack issue, it's most likely one of two problems.

1 ~ A solder joint has broken
2 ~ The jack is worn out

Either way will require precise soldering work on the amp's PCB. If you've never done PCB soldering, I recommend you start with something simpler and take the amp to the shop. If you've never done soldering at all, I suggest you learn how but still take the amp to the shop.
ESP LTD EC-256 and a Fender Deluxe VM