In the mid to late eighties, before I discovered the traditional Mass again, I tried to read her diary. I couldn't. I wound up giving it to an elderly nun. In the early nineties, after I learned all the chicanery that had transpired in the sixties, I bought another copy of her diary. This one I threw in the garbage at work. In the later nineties, I bought another coppy still thinking that there must be something wrong with me that I was unable to read it without thinking that there was something wrong with Faustina. I kept this copy for reference, but still have my own opinions about her. The host flying into her hands always gets me.
I do not recall the SSPX ever making a big deal about Divine Mercy Sunday, and definitely not the CMRI. It's a novus ordo biggie only. Plus, according to Dom Gueranger's Liturgical Year, "Such is the solemnity of this Sunday [Low Sunday] that not only is it of Greater Double rite, but no feast, however great, can ever be kept upon it."
Apparently, JPII was a law unto himself.