Can mylohyoid exercise lift the hyoid bone and reduce an oblique/double chin neck angle?

Thank you so much for this question. The mylohyoid muscle is one of the superficial neck muscles that you have, this muscle helps with the ability to speak and swallow. Unfortunately even doing mylohyoid excercises will not help lift the hyoid bone or give you desired neck angle. However with no pictures available it would be hard to determine what would be needed. Options are still available, you may want to consider 3Deep is a non invasive skin tighting that uses radio-frequency and helps stimulate the bodys natural collagen. ThermiTight is a minimally invasive procedure that requires a few tiny openings and minimal down time and is a non surgical alternative for a face or neck lift. It also uses radio-frequency energy to tighten under the skin. Consider these options with  a board certified plastic surgeon that carries these laser. 

 Neck exercises will not improve your jawline. A full set of pictures are required to make a determination about how best to proceed. If there's a low hyoid, there's nothing that can be done about that. If there's fatty deposits above and below the platysma muscle, then those deposits can be removed with surgery in addition to a platysma-plasty. If there's a recessive chin profile present, then consider a chin implant. For more information  and many before-and-after examples, please see the link and the video below

Unfortunately exercising is not going to raise up the mylohyoid muscle and change the position of the hyoid bone. It may be possible to improve your cervicomental angle with subplatysmal fat removal a muscle plication. But without any pictures to assess it can not stated as to whether this is an option for you.