By the way, canon 813 of the older, 1917 Code was relaxed in the 1983 code (c. 906). The new code lowers the standard for a priest celebrating Mass alone. He needs only a good reason (which is the lowest standard in the law).
The old code said “should not do it” (instead of “must not” or “forbidden”) which implied that he could, and we know that this happened in practice. I don’t have the text handy at the moment (yes online, but no time to look right now).
Liturgical laws from the (older) Roman Missal apply to the Extraordinary Form. The disciplinary laws (ie canon law) of the 1983 Code, as opposed to 1917, apply to the EF.