Unfortunately, I can't help you in using Mosquito. I have never used the platform, although it sounds interesting and certainly worth learning. Winter time project for me.
What I can tell you is that the *.conf file will be the configuration file. You will need to edit this file to set up mosquito. The files ins /opt/bin and /opt/sbin will likely be the execuatuables. What each one does, I don't know. Some reading on Mosquito's website is in order to learn how to use Mosquito from the command line (
https://mosquitto.org/). From the files listed above, I would say you have the client installed as opposed to the broker.
For the service-start script, there is not one yet. You will have to create it once you know which one of the execacutables to use and how to set up the .conf files for your needs.
For help on the Merlin start up scripts, you can read here;
https://github.com/RMerl/asuswrt-merlin.ng/wiki/User-scripts
Instead of vi and nano to edit the scripts/.conf file, I usually use WinSCP to access my router and use Notepad++ to edit scripts and other files (like the .conf file). lines in linux text files are not terminated the same as in Windows and DOS, so you can't use Notepad or the like. Notepad++ can save Unix style text files.
Don't get frustrated though. We all started out learning this stuff from scratch, although some forget that when it comes to giving help. For me, I have pretty much come to the end of what I can help you with as I have never used Mosquito.
Love the grave yard pun though... I will have to remember that one.
Cheers