You did have any reply to your questions, so I will try to give you some explanations. But I'm not an expert.
Memcached engine can't support all types of cache in Moodle, because its mechanism is not "perfect". By example, Memcached doesn't have mocking system, so when Moodle want to cleanup sessions key in Memcached, Memcached drop all the cached vars, not just the ones from sessions!
From Moodle Admin page "Site administration > Plugins > Caching > Configuration". The installed extensions are listed ; in the "Supports" column, you can see which features the engine has. Redis and APCu are good choices.
I do that :
- using Memcached for sessions (do the configuration in config.php (search at "Memcached session handler" in the file) ;
- using a Redis server for Application Cache ;
- I don't share caches between nodes from my cluster, so it's not a SPOF (https://en.wikipedia.org/wiki/Single_point_of_failure). The cache s not optimized to the maximum, but If my local Memcached server fails, others Virtual Machines continue to run.
I don't use settings "$CFG->tempdir" and "$CFG->cachedir".
I don't know how to configure Request Cache, because I don't know any engine for that!