I had to deal with all kinds of issues like the above when working on people's guns, often seemed to be something new pops up (such as an airleak) that takes time to sort out. Custom wiring jobs were a nightmare, and it's true the MP5s take a long time to disassemble/reassemble, so literally a full assemble is required to simply test fire to see if the last attempt at a fix solved the problem. If not, back apart it goes.
I think the OP was expecting the work to be a quick and simple thing, sometimes it is, no more than a couple hours tops, but when you get finicky parts and troublesome guns, it becomes a nightmare for the person working on it, and updates on "This isn't working out with this part" are commonplace.
I'm so glad I got out of the airsoft technician business.