The problem is that most AE groups don't last over 20 seconds or so, so that theoretical performance peak isn't actually relevant. And of course DW frost can AE while barely deviating from its single-target priority!
Your second point is not supported by data. In fact, it's almost exactly the opposite of what you say. Anything below 10% wait time or so is essentially completely GCD-locked.
DW Frost T14H: 3.79% time spent waiting on resources
2H Frost T14H: 5.76% time spent waiting on resources
Unholy T14H: 14.40% time spent waiting on resources
As you can see, DW frost is already completely GCD-locked, while Unholy is just fine. This is because DW frost uses the masterfrost priority, which involves spamming Howling Blast and thus has very few free GCDs.
2H frost is completely GCD-locked too. It's slightly looser than DW, but since it reforges to haste the GCDs freed from following the standard hastefrost priority rather than masterfrost are largely lost.
Regarding weapon scaling, frost does scale with weapon DPS roughly twice as well as unholy, and that could cause scaling issues further down the line. It's definitely a problem for Unholy; always has been.