Stealth is rolled when a ship nominates one or more attacks against a target with stealth.
The energy mine is not fired at that target - it is fired at a point in space. The energy mine explodes and does not need to roll stealth to make an attack.
It is entirely possible for a G'Quan to fail its stealth roll when attacking a Sharlin, but hit the target with its e-mine and so provide the +1 bonus for other ships to beat stealth when firing at the Sharlin.
Breaking stealth can be a pain, it needs careful forethought and planning. If you use fighters to drop the stealth of one ship - don't be dropping your emines on the same ship. But using fighters against minbari can be throwing them away, so make sure you have more fighters than he has AAF. And so on...