It definitely helps quite a bit, especially in longer fights, but Yamaraja's combo down can somewhat substitute that and the faster things die, the less damage they can do. In the end it's kinda situational, if you need to turtle something out, amaterasu is the better choice, if you can just kill it, then mars is.
With Osiris you'd usually play Shingen to take advantage of the frequent bursts. Question is if you really need more heal than amaterasus regen and osiris heal. Spike damage output would be a lot higher.
From the kh you listed, my choice would definitely be tsukuyomi, since she's like the 1 core ssr you'll always use in light, no matter the build. But I might be a bit biased, since light's always been my strongest