RSRH: I think you’re making a mistake when you treat “oiver” and “liable” as interchangable terms. You are not liable until you complete a 3-minute period (thought it was 8 but that’s a separate discussion) but in general we would consider even one minute to fall under the rubric of chatzi shiur assur min hatorah. So if you’re already going to try and play God’s accountant by adding up all the issurim, then you can theoretically count every planck unit as a separate lav, but I doubt it makes any real difference.
gavra: Are you sure that the machlokes RaMBaM and Rabbeinu Yonah is even a machlokes at all? Could be they both agree but from a perspective of tikkun hamiddos and sur merah it would be more useful to look at it the way that Rabbeinu Yonah says, while when it comes to halacha l’ma’aseh even he would pasken like the RaMBaM.