The oil almost always looks and gets a little thicker, darker, and dirtier looking at the bottom as it tends to settle out the bigger particulates as it sits. That oil does not look that bad to me. The fact they are saying your bike had metal floating around in the oil when it does not would be enough to make me either start looking for a new shop or start doing your own maintenance. That shop sounds like it it trying to drum up some business....at YOUR expense! It really is not that hard....ALL of us had to start somewhere, and back when a lot of us did, we did so with ONLY shop manuals, help from fathers, and or friends,....and a little common sense. NOW with the internet we are all able to solicit help, criticism, good AND bad advice, but most of all keep our scooters running while learning things and hopefully saving a pocket full of $$! I would steer clear of that shop FOREVER ........or till they get new management....which probably will not be to long if they are pulling that kind of garbage on customers. I think you are on the right track....check the valve clearance as you do not even have to tear anything down to do that, just pull off the valve cover, then make a decision on what to do from there. I still think it will be something simple due to the way it died on you. No compression for the most part can only come from a few things like ....compression release out of adjustment, bad ring/s,or out of adjustment or bad valve/s. Since it was runningand just quit I have a hard time believing it is rings or valves, but it could be. Or possibly the timing jumped....... but that is not very common either. ALWAYS try to start with the simple things. Nine times out of ten THAT is where you will find the problem! Good luck, keep us posted.