I'll give the advice I would give to a shop worker. I cannot comment on aviation.
1) Ensure you meticulously keep all records and of all conversations that have gone on;
2) Follow the company policy, including the grievance process, making sure you are accompanied at every meeting.
3) Enlist professional help - I assume you do not have a union - so from another point of assistance (do you have any legal advice or protection policy - even house insurance legal protection often covers employment issues?).
4) If you have a regulator - speak to them, and confess all. If your industry is regulated, they are obliged to take an oversight. Technical areas also often have 'operations inspectors' to ensure safety standards are enforced. This way if there has been any wrongdoing, the regulator also has his 'neck in the noose'
Line checks are supposed to be about safety of operation, so a regulator must have a vested interest.
5) If you suspect this is game playing / politics, then let them play their silly games, and don't ON ANY ACCOUNT feed them bullets to use against you. Ultimately if they are wrong and you are innocent, one way or another that will be proven.
6) Sounds like the person concerned has been royally stitched up. Even if they are not in a union, perhaps the union in question would be interested in what this company is doing. The umbrella statute is probably the legal right to be a trades union member. They might not have to recognise a union, but I am pretty sure (armed forces aside) you are within your rights to be a member of a union.
7) If it's not normal to have 4 line checks in such a short period of time - how do they explain this? You could win the jackpot if you can prove discrimination (remember that word). Damages for DISCRIMINATION are unlimited. As you are legally entitled to be a trade union member, and they have done this to you, that is DISCRIMINATION in a nutshell.
I assume you are in a developed country, not North Korea?
If we don't here from you again we will assume you are in North Korea.