Whilst I can see and even accept the logic behind the analysis that within the confines of the game itself, Halberds are not the problem, it is EMPIRE Halberdiers that are, I still prefer to stick with a more conceptually sound remedy as Halberds are meant to be dual purpose weapons as a whole, although historically the length of the weapon varied quite a bit between different periods and users. With this in mind, I don't see why you can't have an 'Empire Halberd' that gives you greater tactical flexibility as opposed to the rest of the armies which would make them too powerful (Chaos Warriors come to mind as have been suggested before)
Or better yet, a special rule on Imperial Halberd Drill that lets them fight in two ranks and have the front rank gain the +1 strength as proposed in the OP. I also support the option of heavy armour and get rid of the shield option as the shield is useless, its a long heavy two handed weapon!
As for this invalidating spearmen, well lets look at spearmen, they just don't belong in an Empire army full stop. They're a poor man's pikeman. So why not update spearmen to pikemen? It fights with the theme of the army as well as the time period. Fight in 3 ranks like a phalanx! Existing spearmen users can leave their models as is or convert, but their unit just got better. Although a trade off would be no shields and add heavy armour instead.
I'm sure that we can reimagine how spearmen are used (skirmishers? throw them!) as a light infantry unit that is cheap with no detachments or only as detachments (skirmishing detachments? hmmm would that work?)
Just some thoughts