Projectile. Not projectiles. Each grenade is its own explosion, so you resolve each independently. However, when their blasts overlap, they combine into 1 DV/AP instead of staying singulars. There is nothing there that says they combine into 1 explosion as far as reach is concerned.
You claim the section you quoted earlier proves you right, but that one literally says you treat it as 1 DV for Damage Resistance. It literally says nothing about range.
If you can quote a rule section that literally says you combine the explosions into a single one as far as damage dropoff is concerned, then your quotes have merit. Right now you're only quoting sections unrelated to that, despite your claims otherwise.