Bear with me, minimaths, I'm going through your post with my critical head on. I'm not making points, just thinking on thread.
the section 71 offence will require an element of coercion or deception between the defendant and the victim
There has been deception, in that claimants are asked to agree - verbally or in writing - that they will be open to offers of training or work experience. This agreement is then used as consent to "volunteer" for workfare. Since only verbal agreement is required, a JC+ advisor could make it up if agreement wasn't given.
So you've got misrepresentation there. However, the terms of JSA and WRAG have always been that you will make every reasonable effort to find work and carry out plans agreed with your advisor (JSA) or attend work-related activities as recommended by JC+.
~ So, on that point, the dispute would be about whether workfare constitutes reasonable preparation for a job. (I suppose that's why they keep banging on about young people and long-term unemployed.)
The financial penalties for non-compliance are coercion. But a normal employer would sack you for non-compliance.
~ So the question here is: Does the DWP employ benefit claimants? This is really important, for other reasons below as well as this one.
The worker being given false information about the law and their employment rights
Employment rights do not, as far as can be told, apply to workfarers. This is apparently because they have no employer.
But if the DWP is their employer, it should be up to the DWP to ensure their rights are fulfilled. They give a sheet of requirements to the provider companies, who give a sheet of requirements to the labour user (supposedly) but those requirements are minimal at best. Moreover, workfarers who complain about their rights being denied are threatened with a non-compliance report.
~ I don't know which laws that scenario abuses, but am sure it's illegal. IF the DWP is the employer, it's in serious dereliction.
Not being provided with safety equipment and clothing, and/or being charged for the provision of such equipment
This is in the terms of engagement (DWP & provider), I've seen it. However, we know it's not being carried out in practice by Jobcentres, providers or users.
~ If the DWP has mandated the provision of equipment, who's at fault when it isn't provided? Whose responsibility is it?
not paying the full tax or national insurance contributions for the worker
The DWP credits claimants for NI. It also supplies tax statements.
~ That seems to prove that the DWP IS THE EMPLOYER.
.... I was going to go through your whole post but I'm knackered!
Point seems to be: As the DWP is now employing people rather than managing welfare, at what point did they agree this with their new employees?
~ Is the DWP not contravening several rather important points of employment law, contract law, trafficking law and habeus corpus?
If the DWP is NOT the employer, how does it justify paying public money for the subsistence and NI credits of workers? Does the public purse get the benefit of workfare? Why is the DWP paying the users to get free labour?
~ Who's getting the financial benefit? And is it legal to profit by such means?
... I was wondering [a] Why no class actions have yet been launched, and [b] How the government proposes to neutralise the arguments, as it must have anticipated them.
(I'm making myself ill with crossness now 
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