Regarding pairing Chrom and Olivia together, yeah you need to keep him away from the other women so he doesn't get enough points for a support rank with them, unless you marry the character to someone else before CH11. Also, Chrom and Olivia do need to get a certain amount of points in CH11 to marry, but that's easily accomplished if you've found some Seeds of Trust. IIRC its like 1 Seed and 2-4 battles gets them enough points. I usually pair them together and send them to that little island with three forts to hold the line against reinforcements there.
Stahl gets married. I've always really appreciated Stahl. He's one of my favorite cast members, even if he doesn't get a lot of screen time (and is thus often forgotten about). Whoever his wife is, I wish for Stahl and his child to get the most out of the relationship.
Stahl's got good physical stat mods and a good class selection to pass down (Luna and Swordfaire!) so he's a decent choice for pretty much any of the physical women. Generally one of the best people tend to think of stat- and class-wise is StahlxCordelia, as Severa comes with Peg Knight for Galeforce and Merc for Hero as a good ending class, and Luna from Greatknight and Swordfaire from Swordmaster really help her. As for who has the best conversations I dunno, on my non-challenge runs I've done I've always put Stahl and Cordelia together.
Optimized Owain. Even considering Lucina, Owain's my absolute favorite child character (for both gameplay and personality reasons) and I'd love to make him the absolute strongest unit he can possibly be. If this can be accomplished without needing Lissa x Denton, I'd greatly appreciate it.
(M)Avatar is basically always the optimal choice as a parent, as long as your asset and flaw don't directly counteract what the unit should do. Him aside, the two best fathers for Owain are Ricken and Henry, depending on what you want. Due to Lissa's -2STR/+2MAG mod she passes down to Owain, his optimal path is a magic one. Henry gives a +1MAG modifier and access to Dark mage for fun Nosferatanking times, while Ricken passes down a +2 Mag modifier and access to Cavalier for Luna.
In general, as long as you don't royally screw up your pairings, you should be fine for Apotheosis. It doesn't require PERFECT OPTIMIZATION to do, and if I remember right, I've heard stories of people beating it with first gen characters only.