Stick with [name_f]Viola[/name_f] (very pretty, btw! :)). YOu can’t please everyone with your choice, but [name_f]Viola[/name_f] is a stunning name that both you and him adore, and I think that’s what’s most important. 
I much prefer [name_f]Viola[/name_f], [name_f]Violet[/name_f] is pretty but connotations with the flower and the expression “shrinking [name_f]Violet[/name_f]” make me imagine a shy child which yours might not be…[name_f]Viola[/name_f] has a strength & lyricism about it which I think is beautiful.
We named our first something that my side of the family hated. It was awkward at first (my mum is the vocal sort, lol) but at the end of the day the baby is yours and your partners, not your mum or sisters. [name_f]Viola[/name_f] is a seriously beautiful choice anyway, they will get used to it.
Voila and [name_f]Violet[/name_f] rate similarly in my book. Both pretty, but simple and not overbearing. Neither stands out as “better”. Both are lovely.
It’s your baby. Not your families. Name her what you and your partner prefer. Your fam will get over it.
I have a friend whose first daughters’ names are [name_f]Violet[/name_f] and [name_f]Scarlett[/name_f]! I believe [name_f]Violet[/name_f] goes by her middle name (which is a variant of [name_f]Eleanor[/name_f]), but it works! My friend actually has a color theme between all her kids though - there is a [name_f]Rose[/name_f], and the boys have color middles. I like it, but it might be a bit much for some people.
I can also see why the double “et” would bother some. I like [name_f]Viola[/name_f] as an alternative, and you and your partner like it, use it! Your family will get over it.
You need to decide if you really love [name_f]Viola[/name_f]. If you do, then you need to love it enough that your family’s opinions don’t matter. It’s wonderful to get positive reactions on your choice, but most people don’t get overwhelming approval. I know we sure didn’t. I have never regretted their names, though. We loved them that much.