Syzygy There is often overlap between the fan games and those official releases; the Namco FC ports seem to have been in collaboration with a fan who reached out but it’s hush-hush. And except maybe Nintendo, in-house programmers are generally going to be using fan-made resources like Nesdev same as anyone.
Yeah, for all the games listed above, there’s a fan component:
Pac Man CE: started off as a fan project and shown at doujin conventions several years ago; M2 worked as a go-between to get it licensed and brushed it up in-house, particularly on the audio front; they’d been sitting on it for years before this release
Gaplus: started off as more of a thought experiment between a certain homebrew programmer with substantial pro dev experience and some people at M2, who simply wanted to see if an authentic Gaplus conversion was even possible
Tetris MD: ported from scratch with reference to the arcade version; the programming was handled by a former Compile programmer, and the audio was handled by a somewhat well-known FM audio junkie who in recent years has been assisting Yuzo Koshiro on certain projects
Darius MD: another fan acquisition, built with the overseas Mega Drive development kit SGDK; M2 helped him brush up and complete the game for the MD/Genesis Mini under the supervision of Taito, and they’ve continued to brush it up even further for the upcoming “extra” cartridge version. (The original fan programmer is a middle-aged gynecologist who just up and decided to start making MD games with no prior experience and he’s startlingly good at it.)
GG Aleste 3 is not a fan work as such, seeing as M2 owns the series, and it has a lot of pedigree: the artists are both veterans of Compile and Raizing, and the director Manabu Namiki is STG music royalty, having worked for NMK, Raizing, Cave and M2. The programmer is the same guy who did Gaplus (who, incidentally, also programmed Super Star Soldier way back when).
M2 also made a Famicom-compliant Steins;gate ADV as a bonus product for a recent game; that one was programmed by the Gaplus/Aleste guy, and the music (which was something composer chibitech had lying around from some other project) is, to my recollection, running in what’s essentially a licensed Famitracker sound driver.