The letters analysis give the view of a lovecraft that is interested on ocultism but is not a "hard" practitioner of it, what the roman dream shows is someone afected by the action of ancient magicians, not someone that have a high magical capacity himself.
People afected by the interaction of high level magicians trend to develop some magical capabilities. And it happens, as is ussual in magic, out of time and space dimensions. So in magic ,that distant effect both on time and space is not so strange.
Being the ancient magic action happened on spain i got a look into the consistence of the facts that the roman dream have ( a lot , and, very concrete and precise, that is not the usuall for dreams) the letter have in great number.
The latin text is correct, well, that is not extraordinary for a literated person of that time, i admit it.
But there are other facts that are very few probable to be known, for example, the situation of the places of the action in spàin close to the west frontier with france, are perfectly coherent. The places are in that part of the country ( some very small places) and the distance from one to another are correct for the means of travel of that time. ( someone point me that from Pamplona to the Pyrenees, 2 days is to much time for a horse travel, but realize that, as the dream says, some of the travelers go on ported chair, that is not as quick as travelling with horse)
In that part of the country there is notice of high level magic activity. In the small village of Bargota is this sculpture depicting the magiciam Johanes, that was a local catholic priest on the XVI century, that got famous by his magic capabilities, as fly, see remotelly, curation, and others. Maybe was one of the last of that magic lineage, that in the dream are called the " miri nigri".
Maybe that all are only circumstancial proofs, but what proof is not circumstantial? Read the narration of the dream that apears on the letters, is one of the best narrations that lovecraft made, and suddenly after some monts, without explanation he decides to hide it, never got published as a narration, why a writter would do something so strange? What he perceived about that history?
The magician Johanes of Bargota.