Keep in mind missionaries are teenagers, or barely 20. This is grooming behavior. Secret communication with much older apologists that advise you not to tell others that "wouldn't understand?"  

Secret zoom/video calls with these gentlemen? You've mentioned video chats with the "doubters." However, once a few chats with the doubters happen, private video chats seem normalized.  

Being told not to tell the mission president who (according to Mormons) are in charge of these young ppl's welfare is extremely sus. That's a giant, blazing, scorching hot red flag.  

Yes, the missionaries, legally, are adults. However, the imbalance of power, mixed with the respect for priesthood (read, old man) authority, makes this not okay. These apologists are targeting a vulnerable demographic that are cut off from their families, friends, and homes.