Indeed, having been able to consult the archives of the SSPX, I realized that Bishop Fellay, then Superior General, had indeed treated the case with all possible care.
At the end of his investigation, and after having heard all the parties, he came to the conclusion that it was not a case of rape, but of a reciprocal sentimental relationship. A very sad and serious story, moreover, since such a thing is directly contrary to the sanctity of the priesthood.
I also learned that other people had noticed at the time the existence of a disordered friendship between the two persons.
This sinful affair credibly explains how several meetings could have taken place in the same place – in the apartment of Miss Kauffman, of which this priest had a copy of the key – under always similar circumstances, over a period of several months.
Most of us called BS on Kauffman's story for the same reasons described here, the nonsensical insanity about Fr. having a key and regularly assaulting her against her will while she didn't take even the slightest measures to make it stop.
This actually underscores that, despite Voris' recent endorsement of the #metoo movement, where every single allegation is treated as truth, sometimes people make crap up and cause damage. Although Arzuaga appears to have sinned against his vow of chastity, a consensual affair and rape are two different matters, and the SSPX was wrongly smeared for "ignoring" rapes.
Was a year of penance a "slap on the wrist"? Not sure, since we don't know all the details. I probably would have suspended him from ministry for longer than that. But it's not as if nothing was done.