u wont see SOF because when u see them, u r already out of your body.
it is possible for a sniper to take out VIPs. however, it is not probable. u have to consider the threat level (which im sure ISD has that figured out).
whether people have access to weapons (no)
how possible is it to have access to "one of those windows", which happen to be offices.
how possible is it for the sniper and his spotter to evade CCTV and security in the building, followed by how they access the offices without tripping alarms and/or showing on CCTV.
as for bombs, it is relatively easy to create explosives and/or incendiary materials. but be rest assured that Singapore is being protected 24/7 by professionals (NSF and Regulars) specialising in this field of work. and, im sure intelligence units from the Army and Police has this area figured out also.
less a suicide bomber in a car, who ram pass the cordon... survived the shooting by our SPF and all the crap and then still manage to detonate the device.
so, just sit back and enjoy the show. the aim of terrorism is to get u to fear a couple of sh!tbags anyway. if they succeeded in that, terrorists win already.