Trimmed the new posts, attached them to the piers with the new hardware, and lowered the deck on top of them. So far, so good, but its only been a few hours.
Time will tell. Rains coming tomorrow, so that should help pack the ground around them (or wash them away).
The new piers/posts... (left to right)
1) kept existing post, poured concrete under and around
2) new post to left of the existing post

3) new post, to left of existing post (now removed)
4) new post in corner, in same spot as previous (cut off)

The whole section
(crooked post with no concrete is the previous post - too much work to remove)

Rough estimate of cost = $250 (pending return of unused parts)
Lowest contractor bid = $500