Exactly. You spelled this out perfectly imo and I cannot thank this post enough. The helmet, covering under the helmet, padded vest and pants and the boots not only served as a disguise but the perp also was likely trying to protect themselves from any defensive wounds from Missy and leaving physical evidence of themselves at the scene. Covering their hair, face and whole body did lower the risk of leaving DNA evidence but it's still possible some was found if the scene was processed closely. I don't think we will know if the killer was successful in not leaving a fingerprint or a hair, or anything else containing their DNA at the scene until an arrest and maybe not until trial depending on what is released to the media after an arrest is made.
If there was a fingerprint found, if the perp has ever had fingerprints taken for any reason (criminal, military, employment) the FBI will be able to cross check it with their database. I just researched this further bc when I first got mine done for work in 2007, I was told the FBI wouldn't store them if everything came back fine. But according to this article, as of February 2015 the FBI is now keeping all fingerprints on file from any employment check, even if you have no criminal background, and will check fingerprints submitted from crime scenes against all fingerprints in the database, including those in the database solely due to a required clearance for employment (here's a link in case anyone is interested in article):
https://www.eff.org/deeplinks/2015/09/little-fanfare-fbi-ramps-biometrics-programs-yet-again-part-1
I do wonder if there was some touch DNA left if the perp had any skin cells or oils, or sweat that could have come in contact with even a tiny area on a murder weapon, on Missy, or maybe while breaking into the church. Dead skin cells and hair (different than touch DNA and more likely to get accurate profile) naturally come off our bodies everyday so maybe during a struggle some DNA could have been left that way. I know touch DNA doesn't always provide enough to conclusively identify a suspect but it can provide partial profiles that have some use if they have some other known samples of poi's to compare to. I have not heard if LE took elimination DNA samples from those who found missy, rendered her aid, and were on scene, but it's possible all of that testing could be a potential hold up in an arrest.
Regardless, the prosecutor is most definitely advising LE on whether or not they currently have enough evidence that would be admissiable in court for a conviction, and what evidence they may still need. They also can compel grand jury testimony if there is one convened now or in the future for this case, and decide if an immunity deal is worth it if the perp had help before or after and the prosecutor feels their testimony is nescessary for a conviction and worth immunity (just a hypothetical, not suggesting anything, I personally think this person acted alone but think some close to them have very strong suspicions). I would also make an educated guess that the prosecutor has watched any significant interrogations that have taken place with potential suspects and could have helped come up with the scope of certain interrogations that might help in court later on. This could very likely be a Capital Case and I think if the prosecutor feels there's strong evidence to prove any of the elements needed for a Capital Case to proceed in Texas they will do so but not until they have gathered and reviewed all evidence closely. Also, someone correct me if I'm wrong but I believe in order to file felony charges against someone in Texas a grand jury must agree that there's probable cause and issue a true bill (felony indictment) so this will be going in front of a grand jury at some point.
Sorry for the long post