EoR Book says that it has a forward turret-mounted twin heavy Ion Cannon although the AoR book says that it requires silhouette 6 to attach this weapon to a ship. Consular - Class Light Assault Cruise is silhouette 5.

Shouldn't that be a Medium Ion Cannon?

Attaching a weapon as an after-market option (i.e. as an attachment) is different than having a weapon come standard on a vessel.

Take for example the A-10 Thunderbolt, which in approximate Star Wars terms is a starfighter (Silhouette 3) that was pretty much purpose-built around an auto-fire capable heavy laser cannon.

So it's not a mistake, much the same as there are Silhouette 5 vessels that can hold far more starfighters than the Hanger Bay attachment would permit, for much the same reason: the ships that come standard with a hanger bay were purpose built to incorporate them, while adding the attachment to a ship that doesn't already include a hanger bay has to squeeze in something that was never intended to be included by the ship's designers.

Plus there's the Oversized Weapons Mount in Dangerous Covenants that lets you do it anyway.

Yeah, the medium turbolasers on the Corellian Corvette run into the same issue. I believe Donovan's advice is the most sound.