A list of mainly material what-ifs to improve the performance of the defenders of the Philippines -
1. Provide greater access to WWI-era reserve stocks to the Philippine Army, in the 1930s. The US provided 250,000 M1917 rifles and a mixture of heavy machineguns sufficient to the Philippine Army TO&E. But there were huge gaps. Notably light automatics, grenades, mortars, mortar ammo and rifle grenades. And, of course, artillery.
- A few thousand BARS, that the US had large reserve stocks of.
- Grenades, unfilled. A grenade filling plant could have been set up.
- A much larger number of WWI Stokes mortars, which the US had in abundance. The Philippines is excellent country for mortars.
- Mortar ammo was a very sore point. A fix to the bad WWI ammo would have required the dispatch of a US Army Ordnance unit to do quality control and remediation of this materiel (and the grenade filling plant, and rifle inspection and maintenance, and etc.) The Philippine Army had a severe lack in the Ordnance category.
- The US could have provided its stock of Vivien-Bessieres (VB) grenade launchers and grenades. The VB's were standard US Army issue in WWI, based on French practice, and indeed it was this widespread use that led the Japanese to develop and widely issue their "knee mortars". Most were adapted to the M1917 rifles. The US did not use the VBs at all, I think, after the 1920s, as the M1917s were not standard issue, and those things just mouldered away. This woukd have gone a lobg way to filling the firepower gap between Philippine and Japanese infantry.
- Artillery. In 1919 the US Army had a vast artillery park. Another hundred or two of unmodernised 75mm field guns would not have gone amiss. And a few dozen ex-French 105mm/155mm howitzers.
To add, helmets, infantry webbing, uniforms, boots, field telephone equipment, military-grade trucks, all of which were short or non-existent in the PA.
All the above were in WWI era US stocks, or were readily available from civilian sources.
To go fantasy a bit, there was sufficient industry in the Philippines to manufacture simple submachineguns of the "STEN" gun class or similar. Similar things were, after all, subsequently made by craft workshops in the country. Add land mines, antitank grenades, etc. A little money spent on a 1930s US Ordnance mission, as above, could have paid off tremendously.
Grenades and mortar bombs could also have easily been locally manufactured.
2. Mobilize and Federalize the Philippine Army six months earlier. The US was already spending vast sums by Feb/March 1941, a bit more in budget assistance to the Phippine government would not have been missed. Many of the problems of USAFFE happened because of the mad scramble of the last minute mobilization, where many men were joining the colors as their units were already retreating to Bataan. They could have had complete infantry and artillery units, at least minimally trained, with officers used to their men, plus many fixes to materiel deficiencies. They would at least have been better than the armed crowd of Dec 1941. An army in this state could at least have contemplated a guerilla-style campaign.
3. The Philippine Army, unfortunately, took the bulk of its officer corps from ex Constabulary officers. As per General Vicente Lim, who was in a position to know, this was a bad mistake. The Constabulary were policemen, and they knew nothing of modern war. They were also way too tied in to local politics, corruption, and a rather ceremonial sense of organisation. What the PA could have badly used in 1935-41 would have been a few hundred US military advisors, many, probably, promoted NCOs, preferably combat veterans of WWI. The PA had no, zero, officers who had served on the Western Front, other than MacArthur.
4. USAFFE was actually engaged in creating a large number of dispersal fields by Nov-Dec 1941. The disaster at Clark Field did not have to happen. My great uncle, a USACE reservist, was called up to do this, build these dispersal fields, with camouflaged revetted shelters, etc. He later died at Cabanatuan. Again, too late. If this had started six months earlier the USAFFE air force would have lasted longer. They would still have been "Doomed at the Start" (Bartsch), but it would have been an improvement.
5. An air raid warning system was being stood up, mostly depending on visual observers. There is a lot of Philippine territory between landfall and anything worth bombing, so this would work better than one would think. The big problem was communications. Again, started too late.