Wouldn't "it is a mortal sin to attend receive doubtful sacraments" be a better wording?
The canon law prohibition applies to both 1) attendance at doubtful masses and 2) reception of doubtful sacraments. Since we're talking about the new mass, I used the term "attendance". It would still be a grave sin to attend a doubtful mass, even if you do not receive Holy Communion. Just like it would be a grave sin to go to confession to a doubtful priest (except in danger of death), even if you are only confessing venial sins. Canon Law's prohibition is to prevent the growth of dubious priests and any attendance/cooperation with such is a grave sin. Such is the danger of doubtful sacraments, that the Church prohibits such in a stern manner.
Putting aside the issue of 'doubtful masses/sacraments' (which is no small matter),
Quo Primum would also make the new mass a grave sin (even "passive" attendance). Because the new mass is DEFINITELY, positively contrary to Quo Primum and therefore a grave sin (even if the priest is a real priest).
Canon Law forbids doubtful masses and also scandalous/blasphemous masses. And also heretical/schismatic masses. Quo Primum
forbids ALL non-TLM rites, which makes the new mass schismatic and a grave offense.
The indult would also be forbidden by canon law, the same way the valid, Anglican masses were forbidden right after Henry VIII. The indult/Anglican (and also the Arian masses of St Athanasius' day) may have been/be valid, but they are tainted with schism/heresy because they are explicitly and openly part of/supportive to the Arian, Anglican, V2 heresies. So by going to the indult, you are saying you are accepting of the new mass, because you are attending a priest/society which is "under new rome". This is quasi-schismatic and a condoning of V2 heresy.