I've taken Lydia with me a number of times to try to defeat Alduin but she always gets killed. I was hoping to leave her at Breezehome and take a mercenary -- probably Stenvar -- who I don't care about to fight with me, but I can't have two of them at the same time. Can I "part ways" with Lydia temporarily, fight Alduin with Stenvar, and then get her back again, presumably at Dragonsreach?

Yes, you can dismiss Lydia and she will return to either Dragonsreach or Breezehome (if you own it).

That's great. Thanks.

You should travel to Dragonsreach, THEN dismiss her.

I've dismissed Faendal (Riverwood) while around Solitude, and he got killed on his way back. I received an inheritance and thought "oh Dear God!"

Not to mention they can get stuck in various places after dismissing them, making them seemingly impossible to find.

So like I said, travel to Dragonsreach and then dismiss her.

blackpete said if I owned Breezehome, which I do, I could leave her there. No?

Yes you can leave her there. But as I recall, when you go to face Alduin, you go by yourself. There is no companion with you. The only other place you fight him is at the Top of the World, where you can have a companion. Then leave Lydia at Breezehome and take someone else. Just use the depart option.

How the heck is she dying? She only dies if you kill her yourself.

Good question. I did not think of that.

When a follower's health gets low, they fall down and enemies don't attack anymore, indeed. However, in the case of the enemy being Alduin I can imagine the damage done per second is so large (i.e. meteor shower everywhere, fire breath, big physical blows, etc.) that a follower could die in one of those large area of effect attacks.

That is something I did not know. Thanks for bringing that up. I have never had a follower die, except by my hand when Lydia always walked in front of one of my spells or an arrow I had just let loose.

http://www.uesp.net/wiki/Skyrim:Followers

The UESP was my source too, I basically play with the UESP open in the background while playing. Whenever I want more background info, I alt-tab to the UESP to find everything to know about something I found out. I love the background info you would otherwise never find out. I don't use the UESP for knowledge beforehand.

I play on the xbox and have a game guide when I get really stuck. lol But the UESP is great.

Guess I've been lucky. Never lost a follower. Don't ask me about my horse though ...