Do you consider it credible that they have received, this year so far, somewhere between 5 and 10% of the entire volume of fiat currency that has so far been put into cryptocurrency in its entire history so far ? And between 10 and 20% since tether was launched? Without much in the way of formal banking arrangements?

I don't really. And the more you look at their activities the more suspect it looks, tether magically appearing in their hundreds of millions to prop up the markets when they look like falling further, it all looks very, very much like market manipulation and lies.