By the rules of Jamming, 'the weapon needs to be reloaded and any ammo in it is lost.' Ho exactly does that interact with let's say a heavy bolter fed from an Ammo Backpack?

I just deduct the value of a "clip" from the ammunition; I think that's 60 for a heavy bolter (?).

One bolt jamming realistically does not and should not destroy an entire magazine of bolts.

We can look at it two ways:

1: The only rounds in the weapon are those currently loaded in the firing chamber and/or the rounds being used in the attack which caused the jam. For a single shot this would be 1 for a bolter, 2 for a storm bolter, and 1 "charge" for an energy weapon. For semi/full auto the number of rounds "in" the weapon would be the max rate of fire for the attack (with all applicable ammo consumtion modifiers: storm, twin-linked, etc.). When the weapon jams only these rounds are considered faulty (akin to real shooters and jams, you don't reuse rounds involved in a mis-feed) and are discarded.

2: The rounds in the firing chamber and those in the detachable magazine are considered "in" the weapon. When the weapon jams, for some reason, all the rounds in the magazine must be discarded. I suppose (though its a stretch) you could say this is a 'mystical' tradition for tech where the source of the machine's discomfort is removed to appease it, but this doesn't explain why non-human or non-imperial factions have to discard all their ammo too.

Personnaly I think #1 makes much more sense.

Aye - you could say that rounds in the magazine do not count as being "in" the weapon, only the projectile actually chambered does. Switch on the safety, detach "clip", cycle the weapon's bolt to eject the fouled round, reload the magazine and load the next round.

For non-projectile weapon this gets a bit more difficult, but I'm sure you can come up with a suitable explanation - for example a las weapon discharging its capacitor with an ineffective wave of heat washing over the gunner's hands and a safety feature preventing another pull of the trigger until you have removed and reattached the charge pack to allow the capacitor a moment of cool air (with the field manual saying you ought to remove any dirt from the connector, claiming this was the most likely reason for the "jam").

1 makes a lot more sense, thanks for that idea.

With las weapons I say that the power pack has overloaded and fused into one big solid lump.

If you're following the interpretation that a Jam destroys the entire "clip" rather than just one shot, then yes, that sounds good.

Or you could assume the safety breaker (like the ones in your bathroom powerplugs) got tripped by a runaway overload (to the weapon, its kind of hard to overload something by taking things out of it) and you have to remove the power source (ammo pack, power feed) before the breaker will allow you to reset it.