I will suggest an alternative to other posts, esp as I use it at least 60% of my OKW games.
Forget about vet on your ISGs. Only thing of real benefit for the first 3 levels is reduced main barrage recharge time, which only happens at vet III. Reduced smoke barrage time won't be making a real difference. Under no circumstances allow inf to get close to your guns, thus reduced RA becomes irrelevant. In any case, they'll get barraged by pack howies, SPGs and rocket arty, costing you the vet anyway at some point.
You only barrage weapons teams and enemy buildings - things which cannot just walk out of the barrage in a second. Team weapons are often neglected and can take a lot of damage before the enemy notices. If you barrage then you do it with 2 x ISGs.
Yes, but I'am still kinda stuggling trying to find this optimal sweet spot distance. It might have a better long range scatter and show better results then regular mortars, but the results I usually get arent particularly good.
This will come with experience, good and bad, even if it means A-moving towards the enemy. You don't use ISGs like regular mortars, they are a superior light arty option. You use them with a unique playstyle. They have weaker AOE profiles so you are relying more on direct hits, which it is good at.
Mortar- Distance: 1/2/3 Damage: 68/40/20
ISG- Distance: 0.75/1.5/2.25 Damage: 68/32/16
Much better for sniping out squads however as they are more accurate. If the target is infantry then you want to encourage them to get into light cover, where they are bunched up and a single shot can wreck their hp. Mortars can miss very often here, ISGs less so.
tl;dr Just keep shooting and stuff dies. Annoy them enough and they will counter battery fire your ISGs with whatever they can get.