How to ensure all deck posts are sharing the load
The idea would be to just adjust all the posts so that they are tight and snug the same amount under the deck joists when the deck dead weight is resting on them. The carrying ability of the posts and beams will come into consideration when later you place a live load on the deck and the joists flex some in response. Keep in mind that just because it appears that beam 1-2 is redundant only gives testament to the fact that there is design margin at play.

Note that the beam 1-2 was clearly installed there because it was needed to deal with the joist span from the ledger to the 1-2 beam. If there had not been a 1-2 beam then the joists would have had to be engineered to carry the load for the longest span from the ledger to 1-3. That may have increased the joist height and cost a fair amount.