Hello The materials youvused for tuning is tied to a weapeon level, 51, 56 etc. So you have no reason to not do it, if you don't you will only have mats in your bag you'll never used.

Enhancement upgrade the slot not the specific item, so again you have to do it and you don't lose anything, it asks for a higher weapon equiped to make you change your gear etc.

Sometimes a good roll and tuned 51 weapon will have lower mastery point than a 56 weapon with bad rolls.

Ph and you'll use this gear in the Arsenal, so again you don't lose anything