I can’t answer the W question, but as regards the strange verb form for “sit”:
First-yodh verbs like ישׁב sometimes lose their initial yodh in certain forms. First-weak and middle-weak verbs also tend to pattern together such that often there are parallel words, one of each kind, or a confusion of homonyms derived from one root or the other and producing similar forms (in this case the other root would be שׁוב, to return).
Sorry I’m a bit late. Can you please tell me where you saw (W)? I have no idea what that is.
Regarding the verb form, שב is the imperative (m,s) of the infinitive לשבת (to sit).