Can a high current trace be on outer and inner layers at the same time?
[ disclaimer: I haven't done parallelization of traces into inner layers.  I'm speaking only from general sense. ]

Yes, you can parallelize traces into inner layers.  

Yes, the traces on inner layers are worse at dissipating heat.  If you want to split the current evenly, the inner traces should be wider than the surface traces.  The calculation in the IPC standard accounts for that.  If you look at the trace width calculator, you'll notice that it produces larger trace widths for inner than outer.

Remotely related article in PCD&amp;F: Trace Current/Temperature Relationships.  Thermal simulation plots.