Occasionally, I enter combat and end up more a spectator than a participant. If my Companion makes the kill do I get credit for the XP or do I lose it?

What is this "XP" you speak of? Never heard of such a thing in Elder Scrolls games.

If you mean your Leveling progress of Skills,
It's not determined on who KILLS the enemy (If it's a Combat skill of course), but rather if you hit them with the weapon accordingly to the skill.

Each time you hit the enemy, the skill goes up a tiny bit, the Leveling progress doesn't wait for the enemy to die for it to go up.

You don't earn experience points in Elder Scrolls games.

Only things your character does will go towards leveling whatever skill is used.

No, it has no effect, since the system does not use XP, as noted before.

You use your skills to level up, and progress is calculated for the next rank at the moment whenever they are used properly (restoration being cast, a piece of armor made or upgraded, blocking a melee attack, etc.)

Example: I'm 5% of the way to getting the next level in one-handed. I swing my favorite weapon at an enemy, and hit him a few times, before he Fus Ro Dah's me off a cliff. I survive, and despite not killing him (and in fact, still in combat with him as he charges down a nearby path to get to me,) I'm now 9% to the next level in one-handed, since I hit him four times. In this instance, each hit gives about 1% progress to the next rank.