Welcome to JustAnswer.com! My name is DriveFast71(Ron) and I will do my best to answer all of your questions completely and accurately.
To remove the ignition coils, you do not need to remove the valve covers. For the front 3 (the ones closest to the front of the vehilce) you only need to remove the coil mounting bolt, then lift the ignition coil off (see inmage #1 below). However, to access the rear 3 coils, you will need to remove the dynamic chamber (see ikmage #2 below) to access the ignition coil mounting bolts.
An oil leak under the vehicle on the oil pan (probably gasket) will NOT casue a faulty coil. It is common for a single coil to go faulty. The coils are usually replaced on a "need-to" basis, and very rarely does more than one at a time go faulty. If you'd like a second opinion- i'd suggest a FREE (except in CA) "p-code" read at any "big chain" parts store, like AutoZone. With these p-codes, you can more accurately and efficiently determine the actual cause. For example- if you get a P0300 code, this is most commonly spark plugs. If you get a P0302 - this woul dmore commonly be a faulty ignition coil for cylinder #2.