Follow the instructions on creating an autoexec service in Kodi by following the instructions in their wiki here…
https://kodi.wiki/view/Autoexec_Service
for the autoexec.py you can use…
import xbmc
xbmc.executebuiltin('SetVolume(75,showvolumebar)')
and you would replace the “75” with whatever percentage you wanted the volume to get set to on startup. Also note that once you create the files you will need to reboot Kodi in order for you to be able to enable the add-on.
As for your remote not controlling volume, that is probably because your using CEC and your TV is not sending those commands to Kodi. This is what normally happens as your TV expects that it should be controlling the volume unless it sees an amplifier on the CEC bus. Since your sound system is not connected that way your TV has no way of knowing and changing its behavior.