One reason that makes me comfident were not one..is we just spent 100k on a player. doubt we would have if we were expecting a big tax bill

There's never been any doubt that Wigan have not used the image rights & employer benefit trusts under Whelan's ownership.
I've been at separate meetings where this has been said.
The people who've announced this have always maintained that if used extensively these financial methods could allow a club 3-4 more players under the cap than a club would have who didn't use these mechanisms.
I would imagine that in the fullnes of time the names will come out for those under investigation and the clubs that are not.

I wonder if it has changed under the new regime at Wigan, given that Tommy had an "image rites" contract at Quins.

the Guardian article clearly says that 10 of the 14 SL teams are implicated. therefore 4 teams are not - of which Celtic are 1, because as you say, they didnt exist.

And the Guardian have got all the facts correct have they?
IL has never mentioned EBT's or IR details.
Given that he's only been in charge 16 months even if he has we won't be that exposed to a tax bill.
Unlike perhaps some clubs where they may be looking at 6 years back payments.