I have a few comments but I don't represent PURE (not Purple) Planet. I'm just a customer.
The article is somewhat misleading, the energy companies that went bust did so because they didn't do their accounting properly. If there was an inability to pay subsidies it's because they didn't allow for it in their financial planning, they didn't go bust because because they had to pay the subsidies, that was just a symptom of poor financial management.
So what you're really asking is, is PP properly managed financially.
I can't imagine for a moment that if PP do have to pay any subsidies to Ofgem, that it isn't properly accounted for.
Inconvenient though it is for the customers whose energy co. goes bust, they are covered by Ofgem so even if PP were to go bust, (and my opinion is that they aren't going to, but then I would say that just by virtue or otherwise of being here) you'd get any owed money back.
If you get a reply back from any of the companies you've contacted, promising that they won't go bust, I'd run a mile. No company is going to give you that assurance.
In the meanwhile you've raised an interesting question the answer to which probably contains an element of commercially sensitive information and which is what is Pure Planet's involvement with renewable energy subsidies.
I'm sure PP and others more knowledgeable than I about the subsidies (which are far from transparent) will be along with comments in due course.
Good luck whoever you decide to go with, what you should be asking yourself is are there are outliers that seem very cheap, and what their business model is. In the meanwhile you might find THIS