I'm thinking of making a menhir savant druid. I'm wondering, can you memorize the domain bonus spells in non-domain spell slots?

I'm thinking these spells are treated like bonus spells with sorcerers....

I haven't found anything that says i can't.

I just got the undine gm boon. I've been looking into what i want to do with this undine.

My idea:
Undine druid (menhir savant archetype) with fire domain spells
+ 1 level of water elemental sorcerer.

My question:
Would this character be able to change the fire spells from the druid into water spells by using the elemental sorcerer's ability to switch any elemental descriptor to water?

Thanks!

Can you take a variant heritage from Blood Of Fiends (div spawn, etc) and that same character has the fiendish sorcery ability also?

From what I'm reading, it looks like the character would still have it!