I kinda agree on dropping the 120mm armed tank. Little need, russian design parameters don't quite fit Italy anyhow - shallow turrets limited elevation choices. Plus, if you wanted an anti-tank type 120mm, you'd need a bigger turret ring than the IS w/122mm, meaning an even bigger tank- and while the bigger dimensions you provided may have served, they would really spiral that weight higher than expected.
Now, for the other discussion :
Semi-Quick Comment on Decapping plates.
The Italian navy specifically called out (so I've read) decapping plates for the Littorio class in real life.
However, with a major shell, the APC cap is something that can be knocked off, and if you have the right thickness of plate....and several feet behind it for the ballistic trajectories to separate, you will have the uncapped shell impact the main belt, and overall penetration will be substantially reduced in comparison to if the armor had been a unified plate.
It's debatable if Tank APRC projectiles work the same way, and with the HVAP rounds they probably don't.
This brings us to the second concept- something I've seen attributed to the Panzer III (as I recall) - which was the front armor was actually 2 separate plates. The first was a extremely hard and thin plate, with a small space before the second plate of more conventional armor. This was meant to shatter the AP nose and allow the fragments to seperate a bit before impacting the second plate. This works up until it doesn't. Vague memory supplies that as face hardening techniques and welding of armor plate became better, this became an unnecessary complication.
Armored Skirts you can see in varying types from WWI when they protected the treads, to WWII where the PzIVH has numerous pics of funny looking skirting meant to gain stand off range as the early RPGs were weak.
Disclosure : Long ago, when I was modifying the data of the Souma S-35 (which provided the picture) concept to be my T-35A, I had the concept of a cast hull with flanges built in. The thinner a casting, the faster & more evenly it cures, so the better the quality and rate of production. This flanges would be "deep" enough that a riviter (later a welder) could attach the main face hardened armor plates to them. The thin casting would then act both as frame and spall lining. A bit complicated, but peacetime the rate of production wouldn't be a huge problem. This became my T-35 write up. Tanksharp came along after, and so I put that into tanksharp and discovered my Dutch tanks are bazooka proof in large areas. So yes, the Dutch have spaced armor, but no, it's not deliberate, but rather a happy accident.