It's the bandits that don't communicate that create a greater risk
Non-transponder and no radio aircraft - and others with radio but who happen to be on other frequencies at the time a broadcast is made - are not bandits; they are legitimate users of the airspace. As you say they create a greater risk, I assume that risk is one identified and mitigated in the risk assessment.
Captainnobars
these guys make broadcast on five different frequencies, of which only two are required of them
Your research clearly hasn't included AIP. ENR 5.5 Parachuting operations states that broadcasts are required on ALL relevant frequenciies. In this case there are indeed 5 or 6 different frequencies required.
Not sure what if anything you are inferring re your discussions with ML TMA, but you might take the time to read the letter to Industry from them at the link I provided.