I managed 5-1 twice, but no chance of doing it five times in a row.
5-1 was by far the hardest because you started with nothing. The later levels you could usually start with a ghost or a shield, if not a weapon to get things started quickly. You surely would have mastered it soon! :)
It's possible to break through the top or bottom of the screen as spikey plobb.
You can break through any edge of the screen with or without the spikey powerup. The trick is to keep the mouse away from Plobb so his turning radius is larger. It's a great way to rack up the points in the bonus level or snag a powerup going off an edge you're close to. Move your mouse closer to Plobb's new position to get him unstuck.
are the top high scores still beatable?
Good point. I could reliably get 600k before the patch. I played through the game just now and came up with 340k points. Unless there's some new scoring mechanism in place that I'm not aware of, the lack of bubbles in world 5 means less potential points.