I met a girl name [name]Amelia[/name] and her nn was Melie pronounced mee-lee. I thought it was sort of odd, but I get where they got it.
I think [name]Ella[/name] is a bit of a stretch, but if it works for you I say use it. What about [name]Lou[/name] or [name]Lulu[/name]?
I love the idea of a [name_f]Millie[/name_f] or [name_f]Mila[/name_f] as a nickname for [name_f]Amelia[/name_f]. [name_f]Ella[/name_f] is a bit counter intuitive, in my opinion.
Not sure if this helps, but I’m considering [name_f]Maizie[/name_f] as a nickname for [name_f]Amalia[/name_f]. I think [name_f]Maisie[/name_f]/[name_f]Maizie[/name_f] could work for [name_f]Amelia[/name_f] as well and it has nice alliteration with your chosen middle name.