This question is partially inspired by the AskHistorians podcast AskHistorians Podcast 151 - Medieval Atheism with Keagan Brewer. Other inspirations for this question include:
The allegation of host desecration has frequently been levelled at the Jews. This is likely due to antisemitic scapegoating instead of genuine cases of Jews going out of their way to desecrate the Host.
This makes me wonder how true the other claims are. Do they have any basis in reality, or are they nothing but slander and scapegoating? For example, were medieval allegations of atheism merely the result of people just trying to ruin the lives of people they hate by accusing them of atheism?
You're asking an extremely broad question – one that contains, I'll note, suggestions of some complex goings-on (black masses require renegade Catholic priests and are assumed to acquire their sacerdotal power via perversion of standard Christian religion, so they are explicitly not "crypto-pagan", for example; if all these accounts were true, they would paint an extremely confused and varied picture of religious belief in the medieval period rather than pointing to the continued existence of one "hidden religion", which is typically the presumption behind much of the ahistorical writing on this topic). So even though this has been an area of interest to me for many decades, it would take more than one person to answer the whole of it properly.
With that said, it would be hard to find a reputable historian who takes seriously the suggestion that major European rulers involved themselves in "obscene rituals" with a specifically religious purpose, or promoted crypto-pagan activities, or that explicitly anti-Christian practices such as host desecration were seriously and regularly practised by any group during this period. There is always more to say, and that very definitely applies to this complex and charged topic, but while you're waiting for fresh responses to your queries, you might like to check out some of the existing resources on this site.
I addressed the allegations regarding William Rufus, the second Norman king of England, and his supposed paganism in an earlier response which you can read here, noting how implausible they are. And I wrote about the unlikelihood of pagan survival in Scandinavia as late as the 14th century in another response which is available here. Moving forward to the early modern period, I covered the realities of supposed "Satanism" in Italy and France in the 17th century in a long essay focused on the "criminal magical underworlds" that existed in major Catholic cities in this period, which you can access here.
Elsewhere on this site, u/Sergey_Romanov provides an overview of current thinking regarding the infamous "Blood Libel". You can read that here. And u/AncientHistory has written an overview of historical "Satanism" and the "Black Mass" that is available here.