I think [name_f]Isabelle[/name_f] [name_u]Florian[/name_u] and [name_m]Jacob[/name_m] [name_m]Marius[/name_m]
I prefer [name_f]Isabelle[/name_f]. It’s so elegant and regal-sounding without that extra level of frilliness that [name_f]Isabella[/name_f] has. I do love [name_f]Isabelle[/name_f] [name_f]Fleur[/name_f]! I also like [name_f]Isabelle[/name_f] [name_f]Dahlia[/name_f], [name_f]Isabelle[/name_f] [name_f]Magnolia[/name_f], [name_f]Isabelle[/name_f] [name_f]Marigold[/name_f], and [name_f]Isabelle[/name_f] [name_f]Wisteria[/name_f].
Does one have a nn you prefer over the other? That could be the tiebreaker.
For example - [name_f]Isabella[/name_f] has [name_f]Ella[/name_f] and [name_f]Bella[/name_f] while [name_f]Isabelle[/name_f] has [name_f]Elle[/name_f] and [name_f]Ellie[/name_f]. (Both of course have [name_u]Izzy[/name_u].)
I’d go with [name_m]Jacob[/name_m] and [name_f]Isabelle[/name_f], just because [name_m]Jacob[/name_m] and [name_f]Isabella[/name_f] are two of the main characters in [name_u]Twilight[/name_u].
Isabelle [name_f]Fleur[/name_f] is stunning! I love the ties to your [name_u]French[/name_u] culture. I feel like it’s a stand out gorgeous name - recognizable paired with a little unexpected. I like [name_f]Isabelle[/name_f] [name_f]Iris[/name_f] as a runner up .
I like [name_f]Isabelle[/name_f] better with [name_m]Jacob[/name_m]. For a start, you’re less likely to get nn [name_f]Bella[/name_f] and therefore the [name_f]Bella[/name_f] and [name_m]Jacob[/name_m]/Twilight connection, plus with a two-syllable brother name the longer [name_f]Isabella[/name_f] just feels a bit clunkier beside it.
Thank you all for your reply!! They were all very helpful, and we were able to narrow down to a few. 
So we definitely like Isabelle better, like a lot of you. (And yes, I totally agree that Isabelle would be a better choice to go with Jacob because I’m personally not a fan of the Twilight series! :p) Here are the middle name candidates:
- Isabelle Florence - very VERY pretty, but I’m not sure about the middle initial “F”… Am I weird, or does anyone else think F looks a bit awkward when you write the first name - middle initial - last name combo on paper? I love it when I spell it out, but Isabelle F. seems a bit odd to me…
- Isabelle Lorraine
- Isabelle Christine
- Isabelle Celine - Too brand-name-y? I’m not too into names like Chanel (it is pretty but not my thing).
- Isabelle Felicity
- Isabelle Rosamund - I love the name Rosamund, but I’m not too sure because most webpages seem to say that the meaning of the name is “horse protection” while it actually means “rosy lips” in German and “rose of the world” in Latin… I do love Isabelle Rose, but Rose as a middle name seems too common these days.
- Isabelle Fleur - Sweet and pretty, but I wonder if people will have problems pronouncing her name correctly? We don’t have any issue within our family, so no problem there, but like with Florence I mentioned above, the F middle initial seems a bit off-putting to me.
- Isabelle Olivia
Which ones do you like best from this list, or would you suggest something else? 
Thank you for bearing with my indecisiveness haha!
I absolutely love [name_f]Isabelle[/name_f] [name_f]Felicity[/name_f] and [name_f]Isabelle[/name_f] [name_f]Fleur[/name_f]! The middle initial doesn’t look weird imo, and [name_f]Fleur[/name_f] isn’t hard to pronounce at all, it sounds just how it looks like it should. All of your combos are beautiful, however, and no matter which one you choose, you’ll make the perfect choice! 
Love both [name_f]Isabelle[/name_f] and [name_f]Isabella[/name_f] but personally I would choose [name_f]Isabella[/name_f] as I just love that extra syllable and also love the nickname [name_f]Bella[/name_f]. And for middle names I love:
Isabella [name_f]Rose[/name_f]
[name_f]Isabella[/name_f] [name_f]Primrose[/name_f]
[name_f]Isabella[/name_f] [name_f]Rosemary[/name_f]
[name_f]Isabella[/name_f] [name_f]Lily[/name_f]
[name_f]Isabella[/name_f] [name_f]Raine[/name_f]
[name_f]Isabella[/name_f] [name_f]Penelope[/name_f]
[name_f]Isabella[/name_f] [name_f]Phoebe[/name_f]
[name_f]Isabella[/name_f] [name_u]Dove[/name_u]
[name_f]Isabella[/name_f] [name_u]Avery[/name_u]
[name_f]Isabella[/name_f] [name_f]Faye[/name_f]
[name_f]Isabella[/name_f] Romee
[name_f]Isabella[/name_f] [name_u]Louise[/name_u]
[name_f]Isabella[/name_f] [name_u]Marie[/name_u]
[name_f]Isabella[/name_f] [name_f]Jade[/name_f]
[name_f]Isabella[/name_f] [name_u]Skye[/name_u]
From your list I love (in this order):
Isabelle [name_f]Florence[/name_f]
[name_f]Isabelle[/name_f] [name_f]Felicity[/name_f]
[name_f]Isabelle[/name_f] [name_f]Fleur[/name_f]
They’re all perfect! I don’t think there is anything wrong with the F initial at all. It’s my last initial so I use it a lot… When I take my partners name I will drop my middle and move it to the middle spot. [name_f]My[/name_f] pre-marriage initials are GDF and after will be GFD, never thought a thing of it!
Rosamund has another [name_u]Twilight[/name_u] connection and is too close to [name_m]Edmund[/name_m] imo.
Isabelle [name_f]Olivia[/name_f] is nice but a bit of a mouthful! [name_f]Isabelle[/name_f] [name_f]Olive[/name_f] doesn’t sounds great to me
Celine, [name_f]Christine[/name_f] and [name_f]Lorraine[/name_f] all seem a bit dated (unless they’re honour names). I don’t think [name_f]Monet[/name_f] fits your style unless again it’s an honour or meaningful to you in some way.
I would also add:
Isabelle [name_f]Faith[/name_f]
[name_f]Isabelle[/name_f] [name_f]Grace[/name_f]
[name_f]Isabelle[/name_f] [name_f]Lilia[/name_f]
[name_f]Isabelle[/name_f] [name_f]Elise[/name_f] 
[name_f]Isabelle[/name_f] [name_f]Alice[/name_f]
Good luck and keep us updated