I believe I understand how to give artes to at least Eleanor. I'm not yet competent enough to make a code using cheat engine so I'll probably just end up explaining what I did for anyone else who might make a code of it later, while trying to learn how to make a code of it myself in the mean time.
In the below screenshots I've given Eleanor all of Rokurou's artes, break souls, and mystic artes, some of Laphicet's, and some of Velvet's stuff:
Some of Laphicet's and Rokurou's artes on Eleanor:
More of Rokurou's artes on Eleanor:
Velvet's break souls, and Rokurou's Break souls on Eleanor:
I tested a few of Rokurou's artes on Eleanor. Those being Form 8: Magma Tower, Bone Breaker, Shockwave Slash, Mowing Blade, Crimson Flash, Hurricane Thrust and they all work beautifully. I have not tested any of the break souls, because I'm fairly certain I'll get some undefined behavior, since I have multiple break souls all vying for the same button.
I hope to have some explanation of how I managed to do this or a cheat in a bit. Mind you, this still doesn't do all I want it to. For instance, I can't set a break arte yet to a slot, like ColonelRVH was able to with Scarlet Edge a few posts ago; and I still haven't determined how to get Zaveid's artes on Eleanor, but I do think that this is a good start.