A quick question...

seeing how the royal guards have the Sidearm keyword, does this mean heavy weapons aren’t only restricted to the weapon on the card and can use other weapons on the Unit Squad Card (both melee and ranged) if possible

I.e could a flame trooper in the snow trooper squad use an E-11 if he knows the enemy is at range 3 as the flamethrower is restricted to range 1?

That is correct, the heavy has all the weapon options that the default trooper has. Except if they have sidearm printed on the card like the electrostaff IRG.