With full sequential you will be able to fire the injector as the inlet valve opens. This means there is less chance for the fuel to fall out of the miniscule amount of air flowing past the valves and so hopefully less requirement to fun rich mixtures to get decent quallity idle and low speed running.
You really don't want to time the injectors to only fire when the valves are open. It's not a horrible idea, but all inall, outside of high or wide open throttle, the benefit does not really show up. For what you say, you do get a good amount of shear off the back of the very hot valve. For that matter, odds are that once the engine gets to full operating temperature, the injector is pretty much flash vaprorizing every injection. I've seen video from quite a few engines that show this.
One of the root problems with combustion instability is spark. The closer you are to MBT, the more robust the combustion will be to a/f errors. So on decel, when most of our engines don't advance the spark with low throttle (or MAP), your spark is a LONG way away from MBT at light loads, so you become very sensitive to a/f errors.
A better solution is to just turn the fuel off (if the system has decel fuel shut off), and then dump a bunch back in when the injectors fire again.
For idle, if you can, advance the spark to closer to 20 deg BTDC, and you'll need a lot less fuel.
There are two core issues with the Alfa combustion chamber- one is turbulance, or the major lack of it- so you don't get really good mixing. The second is the shape of the chamber- with a HUGE surface area and spreading the mixture out a lot (you'll note that the late Bosch cars with high compression had a small island on the top of the piston), it's really hard to get a good robust mixture right at the spark plug to fire at the right time. Better at MBT (if you can find it), but it's still a big, big problem on the older 4 cyl engines, and less of a problem on the V6's (even less so on the later TS motors based on the Nord block).
IF available, I would still go with sequential injectors. But that's what I know, too....