Massive congratulations!
Naming is so hard no wonder your struggling especially as your list is beautiful 
Anyways here are my thoughts on your list…
[name_f]Violet[/name_f] [name_f]Ruth[/name_f] ~ [name_f]Violet[/name_f] is lovely a sugary sweet floral name that feels fashionable and in keeping with your other kiddos names. I love the strong purple imagery attached to this name. I love [name_f]Ruth[/name_f] as the middle name for [name_f]Violet[/name_f].
[name_f]Marigold[/name_f] [name_f]Irene[/name_f] ~ love [name_f]Marigold[/name_f] she’s such a happy summery name that feels bright with gorgeous yellow imagery. [name_f]Adore[/name_f] the nickname [name_f]Goldie[/name_f] and [name_f]Irene[/name_f] is a fabulous middle name. I think [name_f]Marigold[/name_f] feels a little unexpected with [name_f]Lucy[/name_f], [name_u]Lincoln[/name_u] & [name_u]Quinn[/name_u] a tad bolder but works.
[name_f]Mira[/name_f] [name_f]Ruth[/name_f] ~ I like [name_f]Mira[/name_f] and I like [name_f]Ruth[/name_f] but very ‘r’ heavy doesn’t have the best flow I think R can be a bit of a tricky letter when there are tons of them so feels a little off. I think I would scratch this one of your list.
[name_f]Phoebe[/name_f] [name_f]Kate[/name_f] ~ feels incredibly cohesive and almost expected (in a good way) with your kiddos names. [name_f]Phoebe[/name_f] is fashionable, friendly, sweet and bright absolutely love [name_f]Kate[/name_f] as the middle name.
[name_f]Ruth[/name_f] [name_f]Juliette[/name_f] ~ [name_f]Ruth[/name_f] is lovely a classic, strong, timeless, elegant choice with the wonderful meaning ‘compassionate friend’ perfect as she’s the newest friend for her siblings. I like [name_f]Juliette[/name_f] as middle name but prefer the more streamlined spelling [name_f]Juliet[/name_f].
[name_f]Audrey[/name_f] [name_f]Ruth[/name_f] ~ very glamorous honestly [name_f]Audrey[/name_f] just oozes old school Hollywood glamour she’s elegant, sophisticated and impactful. [name_f]Ruth[/name_f] makes a fab middle name. [name_m]Even[/name_m] though I love [name_f]Audrey[/name_f] [name_f]Ruth[/name_f] she lacks the easy going summertime brightness of [name_f]Lucy[/name_f], [name_u]Lincoln[/name_u] & [name_u]Quinn[/name_u]. I wouldn’t put them together.
[name_f]Molly[/name_f] [name_f]Ruth[/name_f] ~ ah it’s odd I love [name_f]Polly[/name_f] and find [name_u]Holly[/name_u] sweet but [name_f]Molly[/name_f] has never appealed to me she reminds me of moles so this combination just doesn’t appeal to me.
If it was me I would want to embrace the happy, fun loving sibling set you have going on and use [name_f]Marigold[/name_f] [name_f]Irene[/name_f] whilst having [name_f]Phoebe[/name_f] [name_f]Kate[/name_f] and [name_f]Ruth[/name_f] Juliet(te) as backups.
[name_f]Hope[/name_f] this helps 

