What is the shiny chance with max(4) shiny charms?

Around what lvl should I start pivoting to endgame strategies and what are they? (Just sturdy metal burst priority?)

Is it possible to reroll lock a yellow item into a candy jar???? I haven't had one reroll yet and I'm at floor 990 and I spend about half my gold each floor and the only candy jar I have gotten (16) from just normal rerolls.

Also is my red shiny Conkeldur a glitch as to not having a shiny color or is it not implemented?

And finally I have adamant Koraidon with passive as my main mon, in what way can I fuse it with Conkeldur to give it the red shiny bonus and keep its ability and nature and preferably not lose stats or would it be worth it? My only other non shiny is a magearna which I use for eternatus cuz I was able to give it calm mind.

Thanks!