Your lack of accurate military knowledge disturbs me.
Anyways,
1. Coil or railguns can have the electric charge they're using to fire adjusted, meaning they can fire at almost any muzzle velocity you want. i.e., they can fire just like a conventional gun, except it's safer than conventional propellant. Fragmentation WON'T KILL THE SHELLS you're firing at. It'll burst, the fragments will miss or simply not harm the metal-covered round, and now you've done nothing but waste ammo.
Oh, and the enemies don't know how to use anything but lasers, so....
2. Well, yes, but now you're into megastructure territory to fire them at a reasonable interplanetary velocity. It'd hypothetically work, yes, but it'd probably be cheaper to just use transports.
3. Yeah but that's not my point. I'm saying we could probably work in another upgrade or two without too much difficulty.
4. OK, didn't know that about tandem charges, though I'm pretty sure they don't just move sideways when detonating, and I'm also pretty sure that it's worse than useless for a sabot-style round, unless these explosives are somehow going to go off in front of the sabot with doing EXACTLY what ERA would do (hit the thing with a shockwave, deform it, and slow it down a bit). Bull crap on the sabot, though. I know EXACTLY how a sabot works, and it's NOT a "MOLTEN" penetrator. Modern-day sabots are generally depleted uranium (if you have access to it) in a dart shape, meant to shoot through armor. HEAT rounds (High Explosive Anti-Tank) produce a molten jet to burn through armor, not a sabot.
And, the best part? You're attempting to fit this all in a 50mm coilgun. In a sabot round, of all things. That's...just not going to work. Your projectile is going to be like three feet long at best. And there's no point doing this fancy stuff for the 150mm guns in space, since they're in a vacuum and it's better to just use brute force/all-the-kinetic-energy to kill things up there.