Chuckle, never underestimate the potential entrenched stupidity of any command structure, military or otherwise. As I say, they definitely have to neutralise Malaya, whether they would change their troop and naval dispositions or not is an interesting question.
Allan has not given us details of potential Naval reinforcements yet but, given the need to help Menzies get Australia on board (granted after a more positive African campaign mind), announcing the reinforcements, both land and sea, will be necessary so in theory spies are not needed. Remember originally the goal of Force Z was deterrance, not to actually have to fight it out.
The Japanese would then know there were armoured reinforcements in Malaya, possibly extra air support and heavy naval reinforcements. Plus with the end of the North African campaign they would know more support is likely in the foreseable future. Even with British reinforcements, it is likely Yamamoto will still both see the US Pacific Fleet as the greater threat and the US in the long-term so knocking it out and either delaying the Americans or forcing them to the peace table will be his initial priority. Pearl Harbour therefore happens on schedule.
As you mention, Japanese sealift capacity is limited at best so robbing Peter to pay Paul is the only way you could deal with the greater British challenege. I suppose it depends on whether you see the reinforced British or the US forces in the Phillipines as the greater threat as that is the only place I can think of you get the extra capacity, maybe Rabaul?