That would be the $25,000 question...but frankly, I'm relieved to say I have no specific person in mind for that older accomplice (who, as mastermind, instigator, and corrupter, would be pure evil). In answer to your question, here is some largely speculative theory:
I juggled a couple theories about accomplice(s) at the outset, but the accomplice-mastermind profile I keep coming back to is an older male, possibly LE-affiliated (or formerly LE), possibly in fitness circles, who is currently involved in illegal drug (and possibly gun) activity. If this older male (OM) is in on this murder with the younger male (YM), it is possible they both have a vested interest in quieting Missy about something illicit she may have discovered (eg. Illegal steroid sales at the gym...which could be just a drop in the bad-cop bucket if this unearths a ring also involved in stolen gun sales).
I can't help suspecting that the reason the TX Rangers and then FBI and ATF got involved so quickly might be that this relates to the current bad-cop investigation in Ellis County. If so, it would certainly explain the almost convoluted vagueness and hush about Missy's manner of death ("puncture wounds" by "unknown instrument" "consistent with tools" etc.) and also that LE will not, even after autopsy, confirm that any one of those tools were the murder weapon, nor that the murder weapon was even left behind. (I think in a plot this thorough that it was not.)
Why did the ATF dogs come back to the CC the day after her autopsy? Did that hushed firearm serial number associated with Missy's murder Monday morning have anything to do with the pawning of stolen guns by bad cop Slaughter who was arrested 3 days earlier? Is Missy's murder related to the current TX Rangers corruption investigation? Is there more bad cop involvement in Ellis County? What other criminal evidence was stolen and sold from the Ellis County property room this past year?
OM (if younger male's trainer and/or supplier) may be in a position to have bribed AND threatened YM into carrying out the murder - by threatening to expose his drug use, or threatening YM's family in some way. OM may also supply (stolen) gun and cash or drugs in payment to carry out murder. YM may have been assured (erroneously) by LE-type OM that he wouldn't get caught, or, if he did, he wouldn't get capital murder thrown at him. YM may also have been willing to carry out the murder if he had a secondary personal grudge against Missy (other than that she might out his drug use), but I think when you're talking capital murder, the odds are greater that more complex motives might be at play.
Gotta go, back later. Thanks for your question - I've enjoyed the discussion, too!