Wow, you really sabo your employer! Start working only one week
& then must go back for reservist? And your boss still don't
know? Tell him ASAP.
Most likely, it's too late to apply for deferment now. I once saw a
civilian boss coming into camp on reporting day & managed to
persuade my CO face-to-face to release one of the men. (We were due
to fly to ROC.) You can try your luck. Better yet, give your boss
your unit phone number so he begin badgering them
immediately.
As for make-up pay claims, check out this site:
http://www.ns.sg/nsPortal/appmanager/nsp/default?_nfpb=true&_pageLabel=nsPortal_NSPAYE
Unless you want to earn only your NSF pay, you'll be filing such
applications for the next 10 years or so, so read the details
properly & know how the system works.