I interpret the term (n. hikikomori) differently to the verb (hikikomoru) from which it is derived.
The term (hikikomori): individual instance of the social phenomenon of withdrawing from society and living in persistent isolation.
The verb (hikikomoru): used in reference to a specific (non-persistent) act of staying indoors, without necessarily carrying the broader sociological meaning. Edit: as another comment points out, just staying indoors for a weekend, etc.
It's basically just staying home when you could do otherwise. A normal reply to "what did you do last Sunday ?", etc.
"hikikomori" isn't a derivation, but a very basic inflection supported by all verbs, used all the time.
Without that you could be "shut in", but that just meant you were inside something with the door shut. It didn't have long-term or social implications.
{Citation needed.}
This seems unlikely given that "hikikomoru" is a compound verb which means to stay inside.
If it's nice weather but someone stays at home all day, the hikikomoru verb is used for that; it (still) doesn't have automatic connotations of it being a way of life.
The "komoru" part has various meanings (according to EDICT): "shut oneself in (e.g. one's room); to be confined in; to seclude oneself; to hide away; to stay inside (one's shell)" as well as "to be heavy with (e.g. smoke); to be stuffy; to be dense", plus others.