From the Altium documentation:
There are two approaches to interactively change layers during
Press the * key on the numeric keypad. Each press of that key will move you down to the next available signal layer.
Use the Ctrl+Shift+Wheel Scroll shortcut combination. Hold Ctrl+Shift, then scroll the mouse wheel forwards to move down through
No, you can't swap DM and DP, it won't work.
SuperSpeed pairs are not routed OK. The wires of a pair are not equal length, it could be better. Typically there would be no ground between wires of a pair.
You can have ground between two pairs if there is enough distance but not really between two wires of a pair.
There are unequal amount of vias, one wire ...
Open the various individual libraries in Altium.
Open the your new target library.
Right-click on the component, in the component panel list, that you want to include in your new library, and select 'Copy'.
Right-click in your new library and select 'Paste' in the component list.
This works for both Schematic and PCB libraries.
Going from Eagle to fAultium ...
It's called "T-Solder" (top) and "B-Solder" (bottom) by default, and you can toggle visibility like any other layer. They're not mechanical layers, they're signal layer pairs, so when you flip a component to the opposite side the solder mask follows.
Here's the top solder mask for a TO-252 in single-layer mode:
And the same part with all ...