My mums favourite movie is the princess bride and I was wondering if the middle name [name]Buttercup[/name] would honour her somehow maybe [name]Eleanor[/name] [name]Buttercup[/name] [name]May[/name] because may is grandmas birthmonth.
Sure, why not! I think it is fine in the middle.
Sure! I think middle names can be whimsical. If you’re going to use 2 middles, I would suggest [name]May[/name] [name]Buttercup[/name] instead of the other way around. That way, if she goes through a phase of not liking [name]Buttercup[/name] growing up, she can just tell people her middle name is [name]May[/name] and leave it off. People with 2 middle names, in my experience, then to use the first middle on forms and such.
If you want to be whimsical, the middle name is the place to do it.
I actually think [name]Buttercup[/name] is cute as well as [name]Bluebelle[/name] 
Go for it!