Fundamentally, the crux of the matter is whether or not Salt's actions were sanctioned by the club and whether or not scouting has been systemic all season.
If it was an intern trying to impress, questions might be asked about the club culture and a fine would be in order, but the club isn't guilty of a deliberate act and the matter should stop there. If the scouting has been systemic, then we probably ought to face harsher punishment.
I'm far from convinced that Middlesborough should get a free pass to the final. There's a limit to what can be learnt from fifteen minutes hidden behind a tree in a field - if you're really lucky you might learn which side strikers favour when taking penalties or which players are carrying injuries - but the counterargument is that having been exposed as a spy, Boro were inspired in the first leg and the spying affair was counter-productive.
If we are kicked out of the play-offs because the spying was a more systemic strategy, Hull should get a free pass to the Premier League. In the event that Boro play Hull in the final, I imagine most Saints fans will support Hull. A more creative penalty would be to allow opposition scouts to watch our training up to match-day, though I doubt the EFL has the imagination to propose this.
Let's propose that this was an unsanctioned one-off and we get to the Premier League by winning the play-offs, there will still be (unfounded) allegations around Tonda and the rest of the team. This may lower the sale value of any players we wish to offload, but may also make it easier to keep hold of key player like Scienza, Peretz and Larin, whose brand will have been damaged by the affair. We can also use this to galvanise the team, as there will be plenty of opposition fans who don't think we deserve to be there. We should use this to build a fortress mentality with something to prove.