Thanks for your question which I have been asked to look at
I am afraid your only redress is through legal channels, however many of these umbrella schemes operated under the guise of what they believed to be compliant with HMRC regulations, which transpired, were clearly not.
Your other issue is this company seem to no longer be operating (if the email has been returned as undeliverable) but I would still urge you to engage an initial consultation with a legal expert in tax, to see if they would be willing to take on your case, but I fear (and this is my honest opinion) that it would be throwing good money after bad.
All you can do is negotiate with HMRC as to the payments being asked for, ensure they are assessing you on the right amounts, and make arrangements to pay, if you are unable to suffer the full cost in one payment.
As for your questions
1) Why was this particular company allowed continue with their schemes
They were not allowed to continue, as soon as HMRC identified, and reviewed the schemes, and determined they were not being operated correctly, they contacted all participants to advise them, by assessing the taxes they found to be due - but you also need to ask, what company when advised they are not set up to legally to avoid tax, would have the lack of morals to continue to offer this to individuals
And clearly the company did not pass on this information, or declare themselves to HMRC, and clearly are baling out now when its participants need them to support them
2) If you knew the scheme was illegal, or had worries about the legitimacy, then HMRC will state that you should have contacted HMRC yourself for clarification, but one has to assume you were attracted by the tax savings to made, and were happy to benefit from this, so HMRC will argue that you had a duty of care, to ensure you were being taxed correctly, and they will state that you knowing entered an arrangement, that offered avoidance of tax (whether you thought this was legal at the time or not)
But you have to remember, that you have had income that was paid tax free incorrectly, and HMRC are only asking form you, what you failed to pay (and should have paid) at the time, and I am very sorry to hear that you have been caught in the crossfire, between what you had been led to believe, and what was actually the law.
and 2) didn't HMRC have a responsibility towards me, the taxpayer?