What if: Force Z (HMS Prince of Wales and HMS Repulse) and a carrier named HMS Indomitable

  I think there was a plan to build up a substantial fleet by ~Mar 42 to defend Malaya and the neighbouring area, as well as more troops and a lot more land based a/c but unfortunately the Japanese struck before those reinforcements arrived.

  Germany was too great a risk once France fell, plus you had threats from Italy and concerns about the status of the French fleet.  At this point while there were concerns about Japan they were largely tied up in China and too distant to threaten any British possessions other than Hong Kong and possibly some Pacific islands.

  Despite loses by late 41 the RN was more powerful while Germany had lost the Bismarck and Italy had been shown to be something of a paper tiger as far as the navy was concerned.  Furthermore with Italian East Africa sown up that threat was removed while also the French fleet looked to be neutralised.  Also Japan was looking far more threatening, especially with the occupation of French Indo-China, which put it much closer to Malaya and the Dutch East Indies.  As such Britain and the empire started thinking about reinforcing the weak defences in the east.  Unfortunately it was too little too late. 

  Basically what was needed was a few more land units, especially with some experienced troops and armour and the sort of air garrison that was planned for Malaya, rather than the much smaller number of obsolete a/c that were actually there in Dec 41.  Its just that with a shooting war in Europe and aid to Russia there wasn't enough resources sent to Malaya because the threat was under-estimated.  Arguably, with the advantage of hindsight, it was the greatest blunder Britain made in WWII.

  Well possibly they couild have sent them a couple of months earlier.  Especially if PoW's wasn't used for Churchill's visit to the US.  However on their own they won't deter the Japanese from striking south and unless their very lucky they are unlikely to be able to do much to prevent the success of such an attack.

  They did actually arrive a few days before the Pacific War started but even if they had been there a couple of months earlier the most likely option is that the Japanese deploy some more forces in the region.

  Possibly.  Presuming she wasn't used for something else.  I have also read the suggestion that, because she was pushed into service for the action against Bismarck, then repaired, then sent to the US on the diplomatic mission the ship didn't do a lot of the working-up programme that RN ships normally did.  Which could have been a factor in damage control problems, especially turning the screw back on which did so much to cripple the ship.  This could have been another problem if it avoided the OTL fate off Malaya.

  Unless there had been some change that boosted the relative position between the allied and European Axis, like the French fighting on from N Africa meaning their fleet was available to the allies and Libya was quickly overcome, securing Africa and the ME much earlier, there wouldn't have been much capacity for earlier reinforcements, even if there had been a clearer realisation of the threat from Japan.  Mind you as I say it really requires ground and air units rather than naval for best effects.

  Well you can get a powerful navy with some butterflies but it would need fairly major changes to do so.  Also:
a)  The cheapest and most effective defence would involve non-naval units.

b)  Even the strongest naval force Britain is likely to be able to send, unless supported by strong land based air support, is going to be vulnerable to Japanese air attack, both b0th from the sort of land-based a/c that sunk Force Z OTL and the Kidō Butai, the Japanese fleet carrier force.  Unless you have substantial pre-war changes the FAA isn't going to have the numbers, doctrine and a/c to fight the KB and win, at least without a lot of luck.