I thought it made sense.
spoilers spoilers
Do not read beyond this point if you don't want to see spoilers
More than one parishioner wanted to kill him because he had declared he would ruin them all and go off to enjoy political and worldly fame success and fortune, which would be kicked off on Easter Sunday (2 days after the murder) with a massive public denouncement of all their sins. He was planning to do this having retrieved the diamond (which he didn't get to do due to being murdered).
The doctor who actually did the murder conceived of the plan in order to execute it in a way where he had the perfect alibi (because he was just part of the congregation during the apparent death - the initial collapse not being the actual death) and in a way to put suspicion on the young priest. The doctor actually stabbed the Monsignor a couple of minutes later when apparently being "helpful" having told the young priest to step away from the body (not actually dead yet) in typical preserving-the-scene pretence. The remote-control burst merely spilled some fake blood at the fake wound site so that it would appear that the stabbing took place earlier than it did.
The resurrection plan, using the cooperation of the grounds-keeper (who didn't know he was cooperating with murderers and didn't know he would get murdered) was for two reasons - firstly for retrieving the diamond - the grounds-keeper thought this was all he was helping with. It was explained earlier in the film that the tomb was built equipped with Lazarus Device - common in Victorian times, where it could be opened quickly and easily from the inside in case you were accidentally buried alive (also common before modern medical techniques) but was very difficult to open from the outside. So by burying the grounds-keeper alive, disguised as the corpse of the Monignor, they were able to get the diamond without anyone else knowing. The secondary reason was because Cy, the illegitimate son of the Monsignor, recognised the extraordinary potential for a very lucrative cult-of-personality megachurch he could profit from with the right story, and footage of the Monsignor rising from the grave (and subsequently conveniently ascending into heaven leaving His Disciples to spread the Word) would be massively helpful with that. So they were planning to dissolve the actual corpse of the Monsignor in acid to support the myth of his bodily disappearance.
Martha cooperated with the murder to save the Monsignor's soul - she had been inspired by the young priest to confess (but to the Monsignor) that she did know where the diamond was. She was horrified to see that his reaction was to grasp at that worldly greed, and thought that she was helping with a plan that would just stop him from doing that and thus would not spiral into the sins of greed and avarice. She did not know that the co-conspirators were planning to keep and use the diamond. When she found out, she became dangerous to the doctor so he tried to kill her, but she realised what was happening so killed him - then took poison herself and confessed all before dying.
end of spoilers
The complexity of the plan was because each conspirator had different motives and priorities and they were each manipulating each other to gain their cooperation in a plan without sharing those motives with eachother. Simpler ways to achieve each step wouldn't have worked because they wouldn't have had the cooperation they needed if they were open about their motives.