They are not charged with being an accessory, they are charged with making threats, assault and obstruction of justice.
None of the above is accurate. But you are right in a general sense, in that there is more information that needs to be added pertaining to the charges.
1 Our discussion has been about the minor characters in this case and the ramifications of the charges against them, and none of them (MP, JP, Dylan, or SA) are charged with any of those charges you list (making threats, assault, or obstruction of justice).
2 Two of the minor characters in this case were indeed hit with accessory charges (MP, JP) as we have been discussing.
3 Beyond that, there are additional charges of which I had thought were part of the accessory charges, but were not.
4 In total, the charges against the minor characters in these crimes are:
MP - accessory after the fact, tampering with evidence
JP - accessory after the fact, tampering with evidence
Dylan - tampering with evidence
SA - none
5 As we have discussed, the accessory charges cannot be prosecuted to a conviction without a successful conviction of JA or ZA or the underlying kidnapping/murder crimes.
6 Yes, the tampering with evidence charges can be prosecuted without an underlying conviction on the primary crimes (kidnapping, murder), although the failure to convict on the primary would certainly have ramifications on whether they were actually tampering with evidence pertaining to a crime. But if they were considered to have tampered with actual evidence, then they could be convicted of that act regardless of the outcome of the trials for the primary crimes.
7 None of the above alters the broader point that ZA and JA are the only ones charged with the crimes against Holly.