I don't even know what to do with the HWT. They are not a part of the formation. Not required.
Maybe it's covered in the rules which comes with the Start Collecting! box. I don't have that.
The infantry squad has the option of swapping two guardsmen for a heavy weapon team. The model is there to allow you to do so - no other reason.
I want to use it as a objective-capping unit so first roll up to the objective with the tank, blast everybody to hell and infantry sweeps the rest.
Fair enough. Bin the heavy weapon, then, but still move the infantry ahead of the tank; they still get the cover save in front, and they'll take a lot more killing by enemy assault units than the tank. 5 Tactical marines charging the tank will probably kill it in one round; it'll take them a few rounds to bring down the squad.
Neither has objective secured, so it doesn't matter which you sacrifice to save the other, and the tank is almost certainly going to be the more useful piece - it has more firepower and a longer range.
Krak grenades for a 'forward squad' are not a bad plan. 10 krak grenades can murder a rhino or chimera easily*, but against something big (land raider or dreadnought) meltabombs on the sergeant will do you more good.
I probably wouldn't take a meltagun because 1 BS3 meltagun isn't really reliable enough - I'd rather stick with supporting bolter and lasgun fire against infantry and leave antitank work to the tank destroyers (vanquishers) or specialists (meltagun command squads, veteran squads or scion stormtroopers).
If you've got multiple armoured shields, on the other hand, having a 'wall' of squads with one meltagun each might be more useful.
* Be warned that apparently a draft
FAQ implies you can only use 1 krak grenade. I sincerely hope this will go away as it's a stupid idea, but take the warning for what it is.