There's surely only 2 ways these contact negotiations could have gone
1) Wray: hi player, come play for us. We won't officially pay you a lot, but we'll give you some of the books money. Keep quiet about it and we'll all win together
2) Wray: hi player. Welcome to Saracens. We'd really love to have you playing for us. In return, you'll win silverware and enhance your career record. We'll also help you setup a company so that you have your own business after your career finishes. We'll invest money in it together. This isn't a salary so is not directly related to anything in your club contract. We've discussed this with our lawyers and premier rugby, and can offer you £XXX as a salary.
Even now that it's come out, it's a grey enough area that none of us are confident that it won't stand up.
As such, as a player presumably being told that it's all above board, that premier rugby and the salary cap manager know all about it, there's very little that a player can do to prove that it is dodgy.
Even if a player does their due diligence, what do they do? They go to their agent who is paid to review these kind of things. If the agent agrees that an "investment" without guaranteed value doesn't constitute an income or salary then I really can't see any player walking away.
On top of that, it's not illegal.
It's all going to be above board as far as employment and tax law is concerned.
The only rule broken is the contract that Saracens have signed with premier rugby.
I can't believe that any of the players didn't think something was a bit fishy, but sometimes in life, good opportunities do come out of nowhere.
Ultimately I firmly believe that it's solely the club that have done wrong. At worst, the players have just sat back and happily reaped the trophies