(2) is possible because AVCHD is effectively the same format used on Blu-Ray discs, but it's a little fiddly. There are several third-party demultiplexer (demux) applications available which can separate the elementary streams from the MTS file on an AVCHD media card and give you the H.264 video and AC3/PCM audio streams as separate files, and matching multiplexer apps that will stitch the streams back together again without altering their internal data. If you can create a replacement for your audio stream which exactly matches the original in bitrate/length/timing then it should work just fine, but it's hardly a one-click workflow.
As this isn't anything to do with Adobe software and these apps are used to extract copyrighted material from Blu-Ray discs I'm not going to give links - but Google for 'AVCHD demux' and you'll find things to try.