Well, first of all, sending oneās purity and consent is not meant for this kind of situation. Technically it may be correct, but itās not really appropriate. Sending oneās purity is done when one has a valid reason for not being at the recitation. In this case, there is no such valid reason. If indeed it is an abaddha-sÄ«ma in the wilderness, it should be easy for those monk to leave the sÄ«mÄ while the PÄtimokkha is recited. If they donāt, the uposatha ceremony is simply invalid.
Yes, it may be that there are some minor offences involved with this. First, depending on the exact circumstances, it may be that you are doing the uposatha twice in one fortnight, which is not appropriate:
āYou shouldnāt recite the Monastic Code three times per lunar half-month. If you do, you commit an offense of wrong conduct. You should recite the Monastic Code once every lunar half-month: on the fourteenth or the fifteenth day.ā
Then there is the fact that the procedure is illegitimate, which, as you say, carries an offence of wrong conduct:
āYou shouldnāt do illegitimate legal procedures. If you do, you commit an offense of wrong conduct.ā
I agree with @Snowbird that it is important to preserve harmony. At the same time, if there is an opportunity to gently let them know that this is not in accordance with the Vinaya, it may be good to bring it up. It is our duty as monastic to uphold the Vinaya as laid down by the Buddha.
Many years ago, while visiting a forest monastery in Thailand, I was in a similar situation to the one you are in now, except I didnāt recite the PÄtimokkha. I was staying at a Dhammayut monastery where they had a policy that they would not do the uposatha with MahÄnikÄya monks. And so they had me and my fellow monks sit within the sÄ«mÄ but outside of hatthapÄsa. I knew it was wrong, but since I was just a junior monk and didnāt know any Thai, it was impossible for me to say anything. It was a strange and uncomfortable situation.
The Dhammayut originally started their own NikÄya because they regarded the majority of the Thai Sangha (the MahÄnikÄya) as practicing incorrectly. Yet now I was observing Dhammayut monks performing an unallowable uposatha ceremony. Things just go in circles! We identify with a certain lineage because is used to be pure. Yet despite the fact that the purity is long gone, we hold on to the distinction in lineage. Itās absurd. It would be sensible and good for the Sangha to abolish these artificial differences and reunite simply as the one Sangha.
Anyway, best of luck!