I've read through pretty much the entire thread, and I think OP has a huge bias for Lapis, and I like her a lot myself.
OP even mentioned how Chloe would have to be fed 'every kill you can to let her reach level 10 by the time we reach chapter 7' which is complete BS. As someone else pointed out, Chloe has 4 maps, because we aren't including her join chapter per OP, to get exp for level 10 before chapter 7. On every playthrough I've done, Chloe gets to level 10 just as I am finish Anna's paralogue for the first master seal. Sure you could argue it is favoritism to give it to Chloe, but for me personally, she is the only one who is generally fast enough to double and kill enemies and reaches 10 first. This has happened with me not giving her an emblem ring before too.
Now we get to chapter 7, for me personally, Chloe is a newly level 1 Wyvern Knight with whatever emblem of choice. For later playthroughs this is now Sigurd because I tend to start getting ready to bench Louis. At this point in the game, Chloe completely outclasses Lapis in every stat. Lapis here is good herself as well and can handle her own quite well, but that doesn't mean that Chloe isn't just straight better. Yea, there's 3 archers sitting on this map, however, not a single one of them can 1 shot Chloe. As I said, with the way I play now, knowing what I like and how I play, Chloe has Sigurd and can usually sweep up an archer and then hide back around because of the walls, so she doesn't die from another archer + other enemies.
Pretty much from here on out, you get your second seal. Yes you can give it to Lapis, which is fine, however, it is either Swordmaster or Hero at this point for chapter 8 because there are no second seals available till after chapter 8. In either case, Lapis is still outclassed by Chloe for chapter 8.
Like I said, I like Lapis and have made her work. I do no think she is better than Chloe or Kagetsu.