1 - I made multitouch easier to work with by placing a sprite at each touch location and checking for collisions with those.
2 - That's a problem not specific to construct 2. I set positions of the ui based upon the viewport edges expressions, then resize the browser window in preview to make sure it looks good at lots of different ratios.
3 - different devices have different amounts of processing power and optimization for HTML5. The same device, if the carriers would allow people to upgrade the operating system, often could run HTML5 faster. It something developers, not just c2 developers, have do deal with. Some devices just can't handle it.
Previewing on a device over lan generally results in the game running slower than if you actually export to HTML5 and upload it to a server, though, as the preview code has a lot of checks in it that the exported code doesn't have. Also have you tried cocoonjs or appmobi's directcanvas? Those can accelerate mobile games as well.
4 - I haven't tried it, but if it does, you can easily recreate it in events. You might want to make a test to check if it does or not.
5 - Invisible objects can slow the performance of the game, as though they are not rendered, they are still processed by the game's code and taken into account when doing things like checking for collisions.