Hi Thorsten and all, I thought I should not miss anything in my question due to which it became quite long and detailed. I am sorry if the information I posted exceeded the minimum working example limits.
Anyway, just from another fellow natbib user (Sebastián Peña Saldarriaga from univ-nantes ...
I am stuck at a point where the automatic function of natbib that should put the same authors, year publication kind of citation at a multiple citing location like 'van Melick et al. 2003a,b' but it is coming up as 'van Melick et al. 2003 ...