As the Junk Droid as the Droid trait, it can use Shared Experience : this will allow it to focus and move one space.

But the card also mention 'When a friendly Droid or Vehicle is defeated' :
Is the fact to remove and put the droid consider as defeat regarding this card ??

Even if it doesn't, this a really strong combo to roll 2 green dice +1 hit.

During the skirmish, Could I also attach the Scavenged Weaponry updgrade to an already deploy Junk droid ?
If i replace It, As the last figure in the group isn't defeat, the udgrade isn't remove (and not score), so the new one created alreday get it ?

Is the upgrade in ready position or still exhaust ?

Spot Weld does not defeat the old copy. If another copy of the same companion would be put into play, the old copy is removed first. It is simply removed, it is not defeated.

The Junk Droid is not part of any group, it just activates as-if part of any Ugnaught Tinkerer group. You cannot attach Scavenged Weaponry to Junk Droid at the start of a mission, only to deployment cards (and the Junk Droid does not even exist then), and I'm pretty sure the Junk Droid companion does not count as a deployment group (because companions do not activate by themselves) so you cannot add Scavenged Weaponry to the Junk Droid companion later.

Sometime, it's obvious : "When a companion is put into play, place its Companion card faceup" so it's no a deployment group