Flames do not destroy mines. Rifle Grenades only clear mines you can see. Unlike s-mines, they don't have little "Mines are here!" signs.
If s-mines are area denial to you, then you are not building a single patch. You are probably building multiple patches to cover an area.
Soviet mines aren't just put there hoping for vehicles - they are used equally against infantry.
Rifle Grenades destroy mines no matter if they're visible or not. (I'm 99% percent sure, can someone confirm please ? ) And I also think that flamers (at least fht) destroy soviet mines as well.