A couple of thoughts:
1. The Spanish army usually has only one grenadier unit, so it's not a trick that can be applied all along the line. If he has a second Gren unit, then that means he has no cavalry, which naturally changes the French tactics, since presumably the French do have some Cav.
2. I think that some of your frustration comes from the fact that you're using a large BW (2") on a relatively small table. It may be necessary for the guys who play in 28mm to use a deeper table. In your case, for example, the Spanish - if defending on the short edge - only need to cover 24BW of front. If you played in 15mm with a 1.5" BW, then the defenders need to cover 32BW. That's the difference of two full units in Line. Something to think about. You could use a 6-foot deep table, for instance, and keep your 2" BW, now the defender has to cover 36BW.
If you use a 1.5" BW, then every unit in the Spanish core list - deployed in Line, end to end - covers 39BW. Let's say the Spanish player thinks he's fighting 1916 and ignores cavalry and just goes for a Veteran Infantry support brigade. He has a total of 14 Battalions now, meaning he can put 6 of them in attack columns and 8 of them in Line, and still cover the whole front - assuming he wants no reserves. Surely - at some point along the front of those 8 Spanish units in Line - the French can find a soft spot? Especially if they've got cavalry that can scare the Spannies into squares?