1 ) The fundamental problem with T-90 or any T series tank is the loose ammo in the crew compartment , if there is a penetration of turret the molten fragment will hit the loose ammo and will cause secondary explosion in the turret but before that the crew could also die from fragments ?
Very few anti tank weapons are actually as accurate as you might think.
Most will simply hit a random point on the target if they hit at all.
This means the idea of aiming for the commanders position or the gunners position is simply a nonsense. A round can hit almost anywhere and at a range of angles.
Replace the penetrator with a very long pole or spear.
Put 3 people in the tank and start thinking about how small those three people are in comparison with the entire tank.
Be aware that while there might be a few vulnerable parts from the front that the vast majority of weak points are the side and rear of the tank for most conventional missile weapons. The belly and roof are of course even more vulnerable, but hard to aim for in most combat environments.
The point is that most tank crew have flak jackets and helmets that will stop fragments so unless the penetration goes through a crew member then for most penetrations they should actually be OK.
It is a bit like a soldier being fired at with 50 cal HMG ammo... a miss will do no damage at all.
A .22lr round in the face can kill and a 120mm APFSDS round through the tip of your finger is a trivial wound.
Most tanks have anti spall liners that reduce the amount of material flying around inside a tank when penetrated.
2 ) Western Tank barring Abrams carry ammo in turret but they are place in ammo containers which makes them safer then carrying loose ammo in the turret.
The Abrams has a compartment in the rear turret that is walled off from the crew compartment with armoured doors to separate the ammo from the crew. The compartment has blow out doors so if hit the explosion will be directed up and out of the turret rather than into the crew compartment.
The T-90AM takes that a step further and has an autoloader in the turret bustle so the crew is separate from the ammo plus the ammo is still autoloaded so they only need two crewmen in the turret.
But there is still the fundamental problem of ammo close to crew which can explode for many reasons and kill the crew
Think in terms of the bulkheads in a ship or submarine. These armoured walls are designed to keep separate sections of a vessel separate so that fire or water or explosions are blocked between sections.
Explosives only detonate perfectly spherically when in open air. When contained the blast is directed and deflected from hard surfaces.
Think of a rifle.
The propellent in the round in the chamber will try to explode in all directions at once.
However it can't explode back towards the shooter because of the bolt. The sides of the chamber supporting the shell case stop it from expanding sideways.
The direction of least resistence is down the barrel but it has a bullet in the way. The resistence of the bullet is nothing compared with the resistence of the bolt and chamber so the bullet is blown down and out the barrel and the gas expands behind it.
Wield the barrel shut and it is likely the bolt will be the component that fails next.
In the Abrams and the T-90AM it is the roof of the ammo compartment that is designed especially to fail.
When it fails the explosion is directed up and out which greatly reduces the pressure in the other directions and in normal circumstances once the roof fails the pressure drops and other potential weak spots are not put under pressure and therefore do not fail too.
3 ) So the option is to isolate it from crew and put it in rear turret bustle , though turret bustle is not as heavy armoured but it isolated crew from ammo , if the ammo explodes the crew may still be safe as they are located in heavy armour.
4 ) T-90AM will still have the problem of ammo close to crew because even though loose ammo moves to turret bustle , there is still ammo below the crew compartment though protected by heavy armour , but the crew still sits above it.
Real combat experience shows that the most common places tanks are hit is the turret front.
Real combat experience shows that the ammo in the T-72 in the under floor armoured autoloader is safe from penetrations as very few penetrations hit the floor under the turret crew.
Real combat experience shows that the problem is loose ammo and propellent stubs in the crew compartment that start fires that cause explosions in T series tanks.
Adding a turret bustle autoloader and keeping the underfloor autoloader gives the crew 54 odd rounds of ready to fire ammo that is armoured and separated from the crew compartment. If the crew does not feel safe they don't need to load the under floor autoloader if they are happy going into battle with 32 rounds.
In the second conflict in Chechnia they were not happy with loose ammo and went into combat only with ammo in the autoloader, which means 22 rounds so the bustle autoloader means they can take more in even if they don't trust the underfloor autoloader.
Also keep in mind that explosives don't always explode upwards. They could design the floor so that an explosion in the underfloor autoloader could be directed down away from the crew.
It certainly raises the question about the threat of mines... but then most tanks are vulnerable to mines.