I don't know why the Expert button isn't shown. The only thing I can think of is perhaps in the Admin user definition, maybe the "change parameters" drop-down is missing the tick for "user interface level".
In Configuration->Base there is also a drop-down for "user interface level" that probably defaults to "basic". You could try changing that to "expert" and see if it helps.
The configuration files aren't deliberately obscured, but of course, editing them is a bad idea since it is easy to make a mistake.
For what you need, they are in the .hts/tvheadend/dvr/config directory. I'm not sure exactly where that is on your machine, possibly somewhere under .kodi directory? A "find . | grep dvr/config" may help you find it.
The files are weirdly named (do not rename them!) and there is one file for each profile you have (you might only have one). In the file there is an entry called "pathname". Make sure you don't accidentally change the file's permissions or owner if you edit it (do "ls -la" before and after edits) and make sure the trailing quotes and comma are in place. Then you need to restart tvheadend, probably a reboot is easiest.