Nothing illegal was stated by the OP.
The circumstances are pretty coincidental re: intent. But that's neither here nor there at this point.
The current risk is that the refiled petition will also be denied. One must remain in legal status to complete AOS, so it may require consular processing for the bf spouse's petition abroad if that path closes.
Anyway, back to the OP's questions...
They complete a DS-160 and interview. They are honest with any questions asked. What happens, happens.
Having a close relative who is seeking residency via AOS from a tourist visa is suspected to be concern that other family members may do so as well, but nothing is confirmed. In the end, they will be issued the visas or not based on their own merits.
It’s not complicated.
Your parents applications stand on their own merit.
They will tell the truth to all questions asked regarding the purpose of their visit .. to attend their daughters graduation
The challenge for them is providing a solid case that they have ties to their home country, sufficient for the interviewing officer to decide they are do not have immigrant intent
