Do you remember when you last addressed this topic with LE? For example, you shared with us the email you sent to HCSO explaining how to go about getting this type of info. If that was the last time you touched on the subject, do you remember how long it has been?
Has HCSO explained to you why they have not even requested the Apple iMessages? For example, do they not have enough probable cause to obtain a search warrant? If that's the case, that would indicate to me more likely than not, they believe the iMessages may prove to be a significant lead, and they are working on collecting more evidence in order to obtain a warrant, which should lead to release of iMessages.
Another example would be a response like "We cannot comment on that," which may mean they are not doing anything, or perhaps they cannot reveal anything related to the iMessages (whether they've found anything nafarious, nothing suspicious, or they are still in the process of obtaining the info.) for investigative purposes. This may also mean they cannot even confirm that the iMessage info. has been requested.
Also, is most of your communication with HCSO done via email? Email is my favorite means of communication (partly because I like to have a record of everything), and use it often. This is something you really shouldn't have to do, but it may be a good idea to visit HCSO in person, if you haven't already. Even if the Sheriff is not available, at least the detective should be.
My then-13-year-old became the victim of an adult online predator a couple of years ago (don't want to take up too much space, so to make a long story short, the perp was recently sentenced to 12 to 48 months in state prison and is a registered sex offender for the next 25 years). LE's initial response was great, and they took the matter seriously right away. My ex-husband and I fully cooperated with them (my daughter, not so much ... sigh), but it was still extremely difficult to get any info. out of them. Absolutely no updates unless asked. If I left a voicemail, they seldom even called back. There were a couple of instances where I went into their office unannounced, just sat there for 2 - 3 hours until the detective finally came out. I still didn't get much more info., but simply seeing in person the seven- to eight-inch-thick case binder he took out of the office vault made me feel better.
It took a while, but I am grateful for LE and the prosecuting attorney for securing a conviction.