@Roshi there's some truth to what you said, but a lot of the rest of your post is speculative and also ignoring some of the existing evidence.
First off, a 7 goal differential over the course of a season is pretty huge (especially in a shortened season). The Caps only gave up 26 short handed goals last year, 7 goals in a whopping 27% of their grand total. The Caps had the fifth ranked raw % PK in the league. If the Caps swing 7 goals in the positive direction, they're the #1 PK in the league by about 1.5% better than Vegas. If the Caps swing 7 goals in the negative direction, their 5th ranked PK plummets all the way to a tie with Ottawa and Columbus for 20th at a very pedestrian 79%. 7 goal on the PK is
a lot
(There's also a parallel conversation about using raw PK% vs Net PK%, which accounts for how many shorthanded goals a team scores. Chara wasn't doing them many favors in the latter department, and the Caps only scored 2 SHG last year, which drops their 5th rated raw PK% to 8th in Net PK%)
Next, there are metrics that incorporate a lot of what you describe, but some folks around here just eyeroll as soon as you bring them up. So I'm using the "actual stats" and "actual goals" that txpd won't groan about as soon as they're mentioned. Rest assured that the relative stats, QoC stats, and expected stats don't paint Chara in a much better light. But even with just these traditional numbers, we have a control factor to look at. Nick Jensen. Zdeno Chara performed significantly worse than his most frequent PK partner, Jensen. Both of them saw similar total ice time, deployments (def zone faceoffs% vs on the fly%), and match-ups. Yet Chara's GA/60 was notably worse than Jensen's (and Hagelin and Dowd's for that matter).
There is some truth to first-wave PKers having worse stats than 2nd/3rd wave PKers (namely because the offensive team gets a chance to set-up without having to gain the zone if they win the initial draw), but the degree to which Chara's stats are worse than his teammates outpaces that significantly. Especially when his stats are worse than other first wave PKers on the Caps. And the first wave PKer rule isn't even entirely universal (Boston's 1st and 2nd pairings are about equal in GA/60, Whitecloud has a worse GA/60 than the top pair in Vegas, Hamilton faired worse than the top D in Carolina, etc)