1.) First and foremost, if I start a "couch co-op" campaign, am I locked into the split screen mode when she's not playing?
No, you can play solo, but see next question.
2.) If I can play the co-op campaign solo, what are the implications? Any disadvantages to this as opposed to a single player campaign? Like in terms of NPC interactions/side quests/etc. does having two player-created characters in my party limit me in anyway?
Custom characters can't be removed from the party, so you can play solo just fine but the extra character will take up a party slot and doesn't have any dialogue. It won't affect your playthrough otherwise.
3.) Is there an alternative option, such as having her jump in on my solo campaign as desired?
She can drop in to pick up her character at any time you are playing, or even create a character partway through your game.
4.) If the answer to #3 is YES, can she still create her own character or does she simply choose a pre-existing member of my party? If she can create one mid-campaign, is that character locked into my campaign forever?
Yes, once she joins the game, she will get the character creation screen. Then the extra character is locked into your campaign forever.
Being able to remove custom characters from the party seems to be the biggest feedback point, so hopefully Larian adds this in a patch down the line.