Perhaps you forgot that the Bills were actively looking for WRs in the free agency period, due to the likelihood of Hardy being sidelined for the first part of the season. TO was signed a month into free agency, after all the best options for WR had gone elsewhere. Since the Bills were under the cap, and were in the market for a starting WR, the signing made sense.
Regarding the OL, the Bills vastly overestimated the talent they had. Edwards was done in by the total dysfunction of the offense, an ill-fated attempt to run the no-huddle, a pathetic line, and possibly the lingering effects from last year's concussion.