Your first paragraph in your sample needs some work, as generally, quoted dialog should only open or close a paragraph. If there is physical action going on during the quote, it should be contained to the middle of the paragraph. If it happens before the dialog, then the quote needs to be contained entirely to the end of the paragraph and likewise if it happens immediately before.
While dialog is an action, it's generally considered a "Free Action" which means a physical action can take place (so long as it doesn't involve the mouth) in a near simultaneous instant, which is what a single paragraph represents. Talking with your mouth full can happen but distorts the dialog (putting food in your mouth should happen before you talk with your mouth full.).
See the below quote for the changes to your first paragraph:
Adellyn and her partner Theo had never ridden in a limo as luxurious as this. The dress their boss had insisted she wear rode up to an almost indecent height.
“Remind me again why the agency chose me for this mission?” she asked, as she tugged on the dress to force it back down.
Notice how the thought of the Limo and the description of the dress were separated from Adellyn and her adjusting her dress. Technically the line about the limo and the dress should be spread out (why was the limo luxurious. What does the dress look like... beyond it being too short? Describe both. Either way, the narrator would not be able to describe all three actions in a single motion, so they need some separation.
As others have mentioned, you seem to be avoiding the word "said." Which is actually a problem. If you want a non-speaking action (said, asked, screamed, shouted, wailed, ect). Then the dialog shoud end without a comma and a dialog tag and move straight to the immediate action. But, recall that the actions that happen with the dialog is best handled in between the dialog.
Finally, you'll notice I bolded "and" because or is illogical. The word "And" implies both joined phrases are equally true (Adellyn never had been in a limo. Theo had never been in a Limo) while "Or" only requires one of the phrases to be true (Adellyn has never and Theo has OR Adellyn has and Theo has never OR both have never.). Another phrase that could be used and would flow better is "Neither Adellyn nor her partner Theo..." This isn't answering your question, but it struck me as not getting the clear message across.
Observe the changes I made in the third paragraph
“Seems like everyone’s forgotten about Fiona," Adellyn said, with an unladylike snorted. She shifted in her seat again. "She’d look just as good in this dress with her golden blonde hair and tan skin. Plus, she’s used to wearing them.”
“Seems like everyone’s forgotten about Fiona. She’d look just as good in this dress with her golden blonde hair and tan skin," Adellyn said, with an unladylike snorted. She shifted in her seat again. "Plus, she’s used to wearing them.”
In both rewrites, Adellyn is given a said tag as the actions happen at the same time she is speaking. The only difference between them is where the pause in her dialog is given for dramatic effect. Personally, when I dialog, I prefer to lead into the narrative break with the thesis of the quote dialog (everyone forgot Fiona), then follow up with the arguments (The dress would be a better fit for Fiona, and Fiona is more used to wearing this type of clothing and presumably more comfortable.) but there is no hard rule The second rewrite gives an interpretation of the first argument in support of the thesis is something that she has ready to go, while the second argument is more improvised to further assist her singular point, or defelect from an implied negative inference by making it appear more logical (the dress would be better on Fiona implies either a jealousy over Fiona being more attractive or accidentally insulting herself by implying she thinks she looks ugly).
You'll also notice that I broke up your first sentance. The action of the first sentence is the third person past tense form of the verb "To Say" (or "Said" if you want to put a fine point on it.) The subject is "She" or Adellyn and the Predicate of "said" is the entire quote. The description of how she said it ("with an unladylike snort") is an adverbial phrase, describing the way she "said" the quote. None of this has anything to do with the simultaneous action of "shifting in her seat", so we broke it out into a second sentence, to avoid a run on sentence.
You may also notice that in both rewrites, shifting in her seat is ended with a period, and not a comma leading into the next quote. Again. You can not talk by shifting in your seat, so the dialog is not part of this sentence. Rather, the quote is a sentence in full, with an implied "She said" since there is no indication that anyone has begun to respond.
When writing a dialog between 2 people (C1 and C2), you can establish a patter such that you can avoid running multiple said tags by implying establishing a back and forth pattern of paragraphs (Paragraph one (p1) is stated to be C1 dialog. P2 is C2 dialog, and every subsequent dialog paragraph follows this odd/even paragraph." However, you should not get into this habit as any new characters joining in OR third characters already present need to be accounted for and you can not imply conversations with 3 or more characters.