In a minyan (quorum) that is doing heiche kedushe with the (inevitable?) scenario that some mispallelim (ones praying) are following the Rama and waiting till after the chazzan's kedusha to begin their silent amidah while some are following the Sefardi/Brisker minhag (custom) to daven (pray) the first 3 brachot (blessings) together with the chazzan, and assuming that there are not 10 of each group present, is anyone yotzei (fulfilling) tefillah betzibbur (praying with a quorum)?
A partial answer. Reb Shlomo Zalman Auerbach is quoted in Halichos Shlomo on tefila chapter 9 siff 2 as saying "a person praying along with the Shliach Tzibur is counted as part of the ten for the minyan, and even when there are not nine men answering without counting him, the Shliach Tzibur may say the chazaras hashatz."
Footnote #2 explains his logic. "Since this person is praying along with the Shatz word for word, he is no worse than a Shomea K'Onah, and it is as if there are nine people answering.
However this is only true when most of the minyan have already finished their tefila, but if most have not yet finished, lichatchila the Shatz should not start, even though there are ten people answering"
From here at least we can see regarding your situation, if less than the majority of people are saying along with the Shatz, it would be fine. More than that would need a different approach.