If you were to go back and find the Dev response as to why Pining Nocturne does not stack with Addle, I think you'd find that there is a lockout on actions being performed during a spell regardless of inaction in between start cast and end cast checks.

Could that lockout be removed? That's unlikely. Why? Because it was likely locked out for good reason, in the initial game design/engine/structure, and even if they could screw with it (which I don't believe they can), the consequences could be disastrous.

Even if, and this is a HUGE if for the sake of entertaining your assertions, it was functionally possible to remove the coding lockout on actions being performed mid-cast for spells only, there likely isn't a single person still on the Development team who would know how to do it. Straight up. Most developers just work with a toolkit. They're not core programmers. The people who wrote FFXI's engine 10 years ago are long since fired or reassigned. On top of that, FFXI runs with an in-house engine. It's not something like Unreal where they can just find an SDK or a random new employee with experience working with it. The know-how to fundamentally alter anything at the engine level has been gone for a long time.

Don't forget that the casting time reduction limit was raised to, what, 87.5%? 82%? Something outrageously high. The majority of the delay during casting is not between the start cast and end cast checks, but in the mandatory delay between actions.

What's wrong with spontaneity charges?