Shower screen will come off by squeezing the spring with a pair of needle-nose pliers.
There's a plastic clip on the front of the machine, with Caravel written on it; pull it towards you. It comes right out. Then, push the kettle lid aside or lift it off. In the back of kettle, at its "heel" as it were, is a metal clip. Push it back away from you and the entire kettle and group assembly can be lifted up and out of the machine. It will click back into the machine like a cellphone battery
The o-ring fits inside a groove in the wall of the piston cylinder rather than around the piston. It is little more than a rubber band --very thin. But if you remove it, clean the machine well, and then lube the o-ring with Dow 111, it becomes surprisingly robust.
When pulling shots, lift and depress the lever a couple of times gently and slowly, to do a preinfusion and get some water in the cylinder. Then do your real pull. Run several blank shots beforehand to heat the group, especially if your thermostat is set to cut off the heating element on the cool side and the heat has not been conducted to the group.