AARrtgh, you need to slow SLOW down. You have so many questions and by the sounds of it you keep running the bike in between them. You say you are probably going to blow it 2 more times before getting it right, that is WRONG. Again, slow down.
Step one: Start over. Your case match appears to be super duper rough. You want it smooth. It sounds like you switched clutch springs but didn't get any results, do more reading, LOTS more reading.
Step two: Pull it all apart and rebuild. It looks like in the first pictures of the thread you simply cleaned the oil/gunk off the cylinder/piston with some sand paper. That's bad. You've seized the thing and now you need to really clean it up before running again. Polini's can be temperamental, but only if you abuse them. Follow roffmans advice and get the inside of the cylinder cleaned up with some muriatic acid. Get all the smeared aluminum off, no matter what. If its not perfectly smooth, or if anything is still on that nikasil, it is going to seize again. Same goes for your piston. Make sure it is just as smooth where you seized it as it is on all the other machined surfaces. Next verify all (ALL) the mating surfaces are flat. That sandpaper to the top of the cylinder can lead to a leak, so smooth it out. Same goes for the intake. Make sure it is flat. Do a drop test. Follow Wayne's advice and heat the piston so it expands like it would when operating. If it has any hesitation, it is going to seize when you run it.
Step three: Re-assemble. Do all of this right. Make or buy new gaskets. Torque everything properly. Get some brass spacers to put over your studs so that the cylinder doesn't mover around when you tighten it up. Before you run it, time your bike. This is the MOST IMPORTANT thing you can do. An improperly timed bike will make poor power, will give you false plug readings, can cause way too much heat, and worst case can seize your cylinder. Get a timing light, and again, follow Wayne's specs. Skimping on timing can lead to a forever poor running bike and increases the risk of your bike breaking.
Step four: Double Check everything. Before you go blasting down the street, make sure you have no leaks. Did you check your timing? Do it again. Before firing it up, does everything turn over nice and easy. Is your gas new and clean and mixed right. You are supposed to run dino oil for break-in. Debate surrounds the oil ratio and its all silly. Run between 40 and 50:1. Always. The oil ratio is NOT what is causing a seizure. Its something else you did wrong. Or maybe I'm wrong, but I don't think so after thousands of unrestrained polini miles, all at 42.5 to 1.
Step five: Start running the bike. Again check for leaks. Start the jetting rich and work down. Just because you are running half throttle down the street and its running super rich does NOT mean you need to downjet the main. Read up on carburetor tuning. There are many variables, and learn how to isolate them.
Step six: Ride your bike. Remember to continually check that everything is tight, and if you might just maybe kind of think something is wrong, take no chances. Go home (pedaling) and fix it before its an issue. Funny sound are always bad and if it was running really bad then somehow its going great, chances are something is wrong.
Finally, before you do anything, read this
Don't port your cylinder. Again, read up a bunch. Mopeds are finicky as is, and polinis can be even more so. I still believe they are the best moped cylinder, but they take time and patience. If you don't follow the steps and take it slow they will break. Its understandable that you just want to get out and ride, I remember when I first rode a polini, its intoxicating. But remember that if you do it right from the beginning it will require less effort and it will run better.