There's a lot to unpick here, not least the rather binary "all or nothing" approach you seem to advocate which is never a reality when there are competing priorities.
Your proposal to send nothing to Malaya means that the UK must abandon its attempts to deter Japanese aggression. That will only accelerate the onset of war in the Far East, which the UK was desperately keen to avoid. London was walking a tightrope between trying to provide adequate deterrence while not significantly impacting its ability to operate in current war theatres. That they failed does not mean that the policy was wrong, or even that the policy was badly applied.
Your comment about Percival is merging two distinct roles and responsibilities. Percival was GOC Malaya responsible for Army forces. Brooke-Popham was CinC Far East, theoretically responsible for the joint forces in the theatre although the RN maintained a separate chain of command for its assets (a key failing, IMHO).
As CinC Far East, Brooke-Popham was responsible for defence in that theatre. It was his job to evaluate the situation and request the forces he felt necessary to accomplish the tasks assigned to him by London. It was NEVER London's job to tell any theatre CinC how to do his business. The man on the ground understands best his local situation and has the professional knowledge to prepare his forces accordingly. Brooke-Popham developed a solid estimate of the required forces but London was only willing to achieve that force structure in mid-1942. No military command EVER has enough forces...they ALWAYS want more, so the typical approach is to muddle through with whatever forces you have on-hand, which is exactly what Brooke-Popham did.
As to "prepare for a siege simultaneously from landward and seaward," that's exactly the scenario that Percival and Brooke-Popham were striving to do. The problem is that the threat for either scenario simply wasn't there in April 1941. As
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pointed out, there were relatively few Japanese forces in FIC before July 1941, certainly not enough to pose a threat to Malaya. Even after July 1941, there still wasn't a large invasion force in FIC. The actual invasion forces that assaulted Thailand and Northern Malaya in December 1941 came from Formosa. The lack of a clear and present threat led London to think they had more time to deliver the requested reinforcements for Far East Command. The Japanese simply moved faster than anyone expected, showing remarkable operational flexibility to accomplish a large-scale amphibious assault at a range never before seen in warfare.
None of the above suggests that the Brits got things right. There were many mistakes, not least inadequate preparation of landward defences. However, again London played a role in that. In 1941, Malaya's primary job was producing rubber and tin for the British war effort. That work was accomplished by commercial companies which had a loud voice in London dissuading the military from any actions that would hurt production (e.g. building defences in Malaya would be "demoralizing" for the civilians; conscripting locals would take them away from the tin mines and rubber plantations, reducing production of these war-critical commodities).
One of the common issues identified by Percival and Brooke-Popham was the lack of an adequate staff in the headquarters to organize and manage defensive preparations. This may seem like deflecting blame but I can tell you from personal experience that having a sufficiently-sized and qualified/experienced headquarters staff is crucial to getting ANYTHING done in the military. It's the headquarters that levies training requirements on subordinate units. If the headquarters staff is under-resourced or lacking in experience (both of which were true in Singapore), then you won't get adequate direction down to the units. This lack of HQ staff was another shortfall that could and should have been rectified sooner, indeed it was probably easier to fix that than bring in droves of additional aircraft, guns etc.