You are right squad value is the correct metric. It's just difficult to assess so net spend is a hard number. Squad value is subjective.
On the subject of manager bounce, both studies I read recently concluded the same thing, essentially. Managers get sacked after a run of poor form,usually. The studies concluded, with evidence, that the poor form is often clustering and teams will revert back to their normal standard with or without a change of manager.
Opinion didn't really come into the articles.
Take Carrick as an example, given it's relevant to the current discussion. At the start of last season he went 7 games in the league without a win. 2 draws and 2 points from 21. He then went and won 6 on the bounce in the league. It's statistical clustering. We didn't suddenly get better.
That same pattern happens all over the championship. Later in the season, mid January, as I recall we went another 7 games with 1 win and 1 draw. 4 points from 21. Straight after that we went on a 9 game unbeaten run winning 5 and drawing 4, 19 points from a possible 27.
Unless you have a squad that is much better than the rest of the division, which we don't, this will happen.
There are some reasons to sack a manager, current form isn't generally one of them, particularly if the manager has shown he is capable of turning things around.