Thats idea is exactly like mine except:
up to a maximum of 10 land units more than the total number of defending units (just like shore bombard, the casualties may fire their casualty shots)
This is a bit gamey right?
Why not just try it in your next game? Their is hardly any scenario where the defender has 10 land units invading anyway, plus even if you got say 12 units landing (attacking) and 12 INF and ART defending you got 12 @ 2 or less, which is 4 hits on average, which is break even for the fortification as 4 infantry= 12 IPC are going to be lost.
Anything else and its not paying for itself. Thats the point really. Its a psychological deterrent, just like in the real war. However, like anything else in AA it can perform very well, which makes it exciting.
Otherwise your point is great.