I believe it was you to whom I just replied in Kodi's forum. There you were not specific as to your reasons for not using the PVR client, but here you are.
As to 1, season/episode information was previously not implemented. It required both an update to Kodi's PVR client as well as an update to Tvheadend (as previously this information was not available over HTSP). If you are using Tvheadend 4.3, and Kodi 18, then you will receive season/episode information for your recordings. Earlier combinations, however, do not have the combination of features necessary to support this.
For 2, it sounds as if you updated your Tvheadend install, but did not properly migrate your recorded shows. In Tvheadend each recorded program is read from a file (a separate file for each recording) at program startup. To get your old recordings into a new install, replicate the file format for an existing recording for each of the old recordings you want to have imported. (There are also several threads in this forum about achieving this, too.)
If you are still determined to use your recordings outside of the PVR client, there are two options.
1. Name your recorded files according to Kodi's conventions so its TVDB scraper will pick them up. The best I have found is to use
$t/$t$-e$-s$n.$x
as your format string. (Note, this will only apply to new recordings, not existing ones.)
2. Use a post-recording script to generate NFO files for Kodi. However, if you haven't figured out how to import your old recordings into Tvheadend, this option may be more difficult for you as you will need to query Tvheadend for the recording details, and then write that to an XML file. In addition, Kodi requires one file for the series itself, and a separate file for each episode individually. This option can be used for past recordings, but you need to manually provide the data yourself.
As to where the metadata for recordings are stored: see explanation for reason 2 regarding your old recordings. Each recording has an associated JSON-formatted that includes all of the recording's metadata, including the filename. It is generated at recording-time, and the files are read into Tvheadend at startup. These files act as the persistent storage for the recording database.