Most MOJ's feature either police suspecting someone right away and pursuing only that person and thus not finding evidence that would have lead to the real perpetrator or not being able to figure out who did it and being pressured to put the blame on someone.
In this case police didn't zero in on Jeremy right away instead they believed his claims that his sister was responsible. So supporters can't claim police went after Jeremy from the outset and missed evidence proving his innocence. The complete opposite happened the detectives believed Sheila was responsible and ignored all evidence that suggested otherwise.
Since they already had someone blamed for the event there was no pressure to find someone to blame they already had the event wrapped up.
Since they were blaming Sheila and were told to look for anything that could establish her guilt the suggestion police intentionally concealed things that established she was responsible can't be made nor can it credibly be argued that they missed evidence of her being responsible given that is what they were looking for.
The police had zero motive to stop blaming Sheila and to instead frame Jeremy. So conspiracy theorists first need to come up with a motive for police to frame Jeremy.
Proof Sheila didn't shoot herself and that someone else did undermined the initial police theory of murder suicide.
Necessity is the mother of invention. A wild tale of police shooting Sheila was invented in order to suggest police had a motive to frame Jeremy. This tale features the claim police needed to conceal they killed her and thus had a motive to frame Jeremy to avoid facing the heat for killing her.
This pathetic claim is all conspiracy theorists have to go on so far as trying to claim police had a motive to be involved in a conspiracy.
Those Jeremy supporters who don't subscribe to this have no plausible motive for police to be involved in a conspiracy. Instead they claim a family conspiracy but their position is a mess because they make disingenuous claims that the moderator evidence could have innocently been transferred by the family or police and know the family would not have had the know how to planted the evidence in the moderator let alone opportunity to remove blood from the rifle barrel and thus dance around instead of discussing such things frankly.
At the end of the day the various nonsense is for their own benefit to allow them to justify believing Jeremy is innocent and they are only fooling themselves with such. Some though delude themselves to much they actually think such nonsense should be able to be used to set Jeremy free. At some point delusion becomes unhealthy.