And, to be fair, I think it's quite "safe" on the smut scale - there's flirting, there's kissing, but I dare say most of it doesn't even fully qualify for making out, let alone actual smut. Still gets you to wonder "wait, did they then go on to do it like that?", but it fits with general PPC weirdness just fine.
And, well, let's get this straight: we have a lot of bad smut appear as the mission's focus in many PPC missions, it would be silly to say that some good not-so-smut shouldn't have a place in the PPC! If we have to nitpick, that's roughly on the same level I kept in some bits of The World Without Authors, which is definitely tame - you hinted at the fact that kind of action was going to happen, without showing it, and that much was more than enough to get the reader to understand they're getting more intimate and less awkward with each other.
That aside, I feel you've neatly filled in a missing link in the chain - with these snippets you really get the progression from "definitely into each other, but still awkward about it" to "definitely a coiuple", really closing in towards the dynamics we're used to see them in as a married couple! It helps that it is an insanely fun piece, too (Dafydd as a merman because Constance decided to turn the tables on him gave me a good laugh!). The only thing that left me puzzled is that you actually went all the way to find seven actual badfics when making them up on the spot would've sufficed! I assume none of them would've worked as an actual mission, though, which imakes a perfectly fine usage case then.
This piece also reminded me of a couple things. The first is that I still have to get working on that "Nikki accidentally ends up in the underwater section of HQ" interlude, and the other is that I did originally plan on not making her and Sergio retire after Blank Sprite and so I would've likely ended up doing some similar pieces myself! Probably more focused on the two of them adjusting to living as a couple more than about the flirting, but it would've surely included them trying to find spare time during a mission to... do things.
Which inspired me to do some 3D shenanigans that are shamelessly plagiarized from liberally inspired by Dafydd and Constance's outfits! Trying to keep them as worksafe as I can, on average there will be less skin showing than Huinesoron's. Also, there's just three of them as not all of those situations fit with something Sergio and Nikki's canons of specialization offer. In fact, these three are a bit of a stretch already...
1) A Whole New Timeline
"So, uhm... What's the idea here?"
"Come on, don't tell me you're not enojying seeing me in a dancer outfit! It will be a good enough disguise for the SEP field to work, and maybe we can try to spare some time for some dancing later."
"So that was you plan... Deal."
As TWWA has shown, Nikki isn't above teasing Sergio from time to time, so in this occasion she chose a belly dancer disguise just for his eyes. Now, it would be quite improbable for them to get an Aladdin mission, but at the same time pretty much any AU fic with a 1001 Nights theme could result in this. So, yeah, I chalk it up as "likely happened" in the "Sergio and Nikki remained at the PPC" timeline.
3) Follow That Boat
"A cutlass! My ARX-160 turned into a frigging cutlass! I hope Corolla fixes our DORKS soon, because rules or not I want to use my guns as such."
"Maybe you can give it to me? I have more experience in fencing after all."
"Yeah, that sounds like a plan. But why did you decide to go as a mermaid?"
"I wanted to have a swim, that's why! Together with you, of course."
Making a pirate-ish outfit for Sergio was trivially easy, and I already had a mermaid getup for Nikki, since she canonically can transform into one. Though this model is the one I recently made for TWWA, after Nikki recovered her powers - her tail is no longer pale blue but green as the mermaid transformation "melded" into Lyrical Nanoha powers becoming her Rare Skill. As such, the tail's scales now take their pigmentation after her eyes', and she now sports gills while transformed (thoug this detail isn't set in stone yet).
They're not in Peter Pan, of course, but likely in some kind of crossover between one of their usual anime canon and One Piece. Or perhaps a Cardcaptor Sakura pirate AU. Name any kind of AU, someone's bound to have made a CCS fic with that theme.
As whether this leads to any "underater action" by the two of them... well, they did a lot of underwater diving during their honeymoon, Keiko was conceived during said honeymoon, Keiko's magic is water-themed. Corolla thinks the last one is no coincidence. Sergio and Nikki never managed to refute the allegation.
6) Hawaii Undercover
"So... We're PPC agents undercover as tourists, tailing a Suvian who fooled NCIS into thinking she's a part of their team, while said team is also undercover as beach bar staff."
"Yeah, it's silly. Not really the typical Hawaii vacation, huh?"
"We're sticking aroung for a proper one after this mission."
"Deal."
While I never wrote any mission in those, Sergio is also "cleared" for several police drama series, NCIS being one of those. And NCIS has an Hawaii spinoff, while there is also Hawaii 5-0 set there. Goes without saying that if there's a place that a young couple from Earth will gladly get liberal with how long the mission cleanup took, that's Hawaii.