Is it possible to install AD on a SP server after the database & sp have been
installed?
Or maybe it is possible to install on another server, if the above is not
possible?
Any help regarding this would be appreciated.

Is it ideal & possible to create a form using infopath (example: applying for
leave) and combining code built using VB (script files)? And if so, how can one
combine these two?

n.b. The final outcome would be to attach an approval workflow to the form once
a user fills it in and submits.