Okay, took me some time, but I got a good fix for the spinning around problem with the grenade launcher.
What I did is set the projectile speed to 33% of the launch speed, and set the trackingStr to "Poor" and speedStr to "Slow", and I added "DIRECT_AIM" to the hints(this hint doesn't seem to do a whole lot to help, but it did seem to help it a little bit. Could just be random luck though.), and now it's being used damn near exactly like it should be.
Not gonna lie, I thought it was gonna be a band-aid fix, but it seems to be a great fix. The slow down of the grenade projectile seems almost the same as before, strangely enough, only slightly reduced, and the aiming and spinning in a 180 problem is fixed. Perhaps not ideal, but I'd say it's a pretty great work around for the time being.
@Tartiflette Now that the grenades aren't being wasted, the grenade launcher is probably a bit more powerful than intended. Slap an expanded missile rack and a pilot with missile skill, and you have 30 grenades with 500 damage each, which is some absolutely insane burst damage for a 25 DP ship, and it's AoE to boot. This may be intended, since it's a one-of-a-kind ship, and it's not overly over powered, and lands within the realm of reasonable, but I just wanted to give you a heads up. Although, the slow reload rate probably balances the whole thing out. It'll do a lot of damage early on, but then balance out. Maybe I'm wrong about it being slightly OP under the right circumstances.
I applied the same logic as I did to the Wunder Wanzer's grenade launcher to the Aleste's grenade launcher from the Armma Armature mod, and it works great now too.
For those that are interested: A note on the Aleste's grenade launcher is that I set it to 33% projectile speed, and it wasn't quite making it to the target, so I bumped it up to 41%, and that seemed to be the sweet spot. So, basically, somewhere around 33% of launch speed, give or take 10% or so, seems to be the way to go for making grenades from grenade launchers behave properly, be aimed properly, and not cause the ship to spin around like a silly goober.