While I can't comment on anything business class related, I flew SIN-IST-VKO just last spring and all was fine really. Not a clue what aircraft was used mind you, but another vouch for booze on board! Indeed, as previously mentioned, bus transfers can happen at IST and there is one hell of a walk often involved, not to mention there are, at least the few times I've been there, Mecca-bound pilgrims everywhere (particularly Algerians for some reason?) but after missing an IST-TLV due to a RUH-IST being delayed by weather both in RUH and around IST back in 2014, actually, it was pretty easy to get on the next flight. Quick and simple transfer of flights from the transfer desk.