I get that, but usually there's no rush to CR a unit, so might as well put it off for much later if it allows you to save several rainbow fairies.
Yes. Like I said, you will have more than 15 usable units in no time anyway. Basically lolix explained what I meant, but here are some random examples: You might need 2 or even 3 heavy armors on some maps even though you'd probably only use 1 on most maps. You might need 4 or even more healers on certain maps. Some maps have only few ranged slots, so you might need melee supports (units you place on a melee slot that can attack at range) and/or priest warriors. Some maps (subjugations) don't have natural UP regeneration, which means you probably wanna use 2 soldiers and possibly some valkyries there.
...etc.
You can steamroll low tier maps with a generic team, but for god tier maps or certain special maps you'll have to adjust your team (unless you have several OP AW'd units maybe).
Honestly, Alissa is pretty decent, especially if you can mincost her later on. To be fair, it's not quite as easy to CR silvers on Nutaku compared to DMM.
Regarding Waltz, I suggest you try to CR her, but don't try too hard (as far as spending SC goes).