Language users often use comparatives and superlatives of adjectives that, logically, oughtn’t to have any. “That’s the deadest cat I’ve ever seen in my life,” just to give one example! Exactly what one’s expressing by using the superlative there depends on the context. In principle I don’t have a problem with, e.g., “rather unprecedented,” “a wee bit unprecedented,” “highly unprecedented,” where the point of its use in each case is supplied by the context and user. But in the case of the OP it’s simply unnecessary. Add to which (imo) “particularly” doesn’t collocate too well with “unprecedented,” although I’m sure one can find uses of it online.
I was going to say definitely “at” and not “in” here (it’s evidently my preference!), even though a particular hour isn’t being specified, but an extended space of time; but in fact both are used. A few online examples:
Every order you place does a little to assist displaced service workers at this unprecedented time. UPDATE: As of this morning, Monday March 23, we have ...
20.03.2020 - Opinion. In this unprecedented time of coronavirus, we need to be there for local businesses. Upcoming Events. By Maribel Perez Wadsworth ...
We would like to thank you for your continued support and understanding at this unprecedented time. Look after yourself, your family and your friends.
vor 3 Tagen - "We are the largest commercial lender in Wisconsin, and we carry with that the responsibility to support commerce in this unprecedented time.
---there's no difference in meaning, "in" and "at" are interchangeable here.