Is there a minimum range where a 'cool' ship (silent running activated) will always appear normal on the sensor screen?
It's about 500m with A class sensors to lock onto a silent running target and ~350m with D.
You'll show up as an indeterminate contact at about four times this range, however.
1) In PVE, the enemy's weapons will be able to track you and hit you better than a CMDR's. I don't care for anyone else's comments on this, I've seen it with my own eyes. Don't want to hear from FDEV about how the NPC's can only do things a regular CMDR can using equipment that functions exactly the same as ours - we've all been interdicted mysteriously by teleporting NPCs, interdicted through stars, etc. We all pretty much know by now that's not entirely true.
Any NPC and any CMDR who knows what he or she is doing can fire at you without having to target you and can track you without sensors if you fly in front of them. There used to be a tutorial mission based on silent running, and it was easy to demonstrate this in a repeatable scenario with this. Why they scrapped that mission (which was probably the best one) I have no idea. Regardless, it's not hard to sneak up on NPCs and follow them around while silent running with them being none the wiser...until you let them actually see you.
Generally, I find low heat or silent running much
more useful against NPCs than skilled CMDRs. The inverse is often true with novice CMDRs.
Now of course this depends on the type of scanner the enemy CMDR is running but I've found that the hardcore PvP folk tend to run only D-rated scanners for weight savings and anyone else running A-rated scanners is usually new enough to be easy to get away from and still disappear.
"Hardcore PvP folk" can visually identify ships at 10-20km by the sort of exhaust trails they leave and the spread of weapons fire. They are also most likely to investigate unresolved sensor contacts, when a novice might ignore something they can't target, until it starts shooting at them.