I've had few days looking over things and have some ideas on how to tweak the system to get closer to what was on Bataan.
31st Infantry- US Rifle Company as is, but you really should drop the Bazookas. I mean, you are entitled to them, but they were never in the '41-'42 Campaign. A semi-fair exchange would be to replace it with the M1919s which WERE in the Philippines. In other books the bazooka is +1 per team, so switching them out is reasonable. They did have the Garand so the basic gun line is fine. The 31st was stripped of experienced officers and NCOs and had them replaced by draftees to the tune of a company being 25-33% new guys, so the Confident/Green is fair.
45th, 43rd, and 57th Infantry (PS)- As above, but I don't think the Confident/Green mix is accurate for them. They were stripped too like the 31st, but their replacements were well vetted, and very well trained. These units were excellent ones and kept their edge throughout the campaign. I would say give them the Tropic Thunder Experienced card. That gives them Confident/Trained plus the 4+ to hit number. That's very respectful and shows them to be the elite that they were. Remember to switch out the bazookas for M1919s. They did have the 37mm anti-tank gun, but only like 9 were present in the whole of USAFFE with 3 per Philippine Scout and US Army infantry regiment.
BTW- no T30s were in the Philippines, so they are a real no-go.
26th Cav- This is a head scratchier. They want you to do the dismounted cavalry platoon with the Springfield option. Yeah, no. They were fully equipped with the Garand, but oddly enough no BARs. You could use the stats presented as all of those weapons, barring the carbine, were in the Philippines.. however the 60mm mortars were only in the infantry regiments and no ammo was ever sent for them! Also the Confident Green just doesn't wash with them. I'd suggest just fielding them as 8 stand Rifle Company platoons with the Tropic Thunder Experienced card to get their stats up to where they should be. The downside is that you get the quality without the ability to spearhead. However as the campaign went on they were more often than not used as elite infantry. Flames of War gives, Flames of War takes away. Also feel free to have Bren Carriers with these guys as the regiment eventually dismounted and used the carriers that the US picked up from a shipment that never made it to Canadian battalions in Hong Kong.
Philippine Army- No changes there. This card is correct as it stands. If you want to show them as more experienced then build them as US Rifle Company and give them the Springfield card.
Other US Army ersatz infantry (Hq, chemical, USAAF, Bridgit's Naval Battalion)- like the Philippine Army. You could argue it out if they were more or less willing to fight. I'd argue that Confident/Green with Springfields works fine.
4th US Marines- They really didn't see a whole lot of action in the campaign. They were Shanghaied to be MacArthur's pretorian guard, so they spent most of the campaign side-lined- much to their chagrin. However a number of Marines 'excused' themselves and got stuck in where they could. That was mainly during the Battle of the Points, but a fair few volunteered for anti-sniping work with the 57th during their opening action. A few were killed or wounded causing command to drag them out of action. I would argue that the US Marines Naval Construction Company is more accurate for them. This unit was a mixed bag that never got to train together in full on force-on-force actions. The Springfields are accurate, but they also had plenty of Thompson smgs and BARs too! They never had the M1919s, but they had Lewis Guns and plenty of M1917s. However I could see an argument for the regular Marine Rifle Company, but with tweaks to their weapons because they did have a lot of automatic weaponry, but didn't have access to all of the toys in the book. Maybe not enough to be rifle/mg teams, but perhaps enough to avoid the slow firing penality. They also had quite a number of M1916 37mm infantry guns, but no 37mm anti-tank guns. They also had 81mm mortars, like the Army, but they had more of the proper ammomunition for it. In fact, a weapons platoon with M1917s and 81mm mortars were sent from the Rock to support the Navy Battalion during the Battle of the Points.
Artillery- Do the 105mm with the 25lbr downgrade. It's not spot on, but it's close enough to represent the 75mm guns that were the mainstay of USAFFE's artillery park. Also take the 155 up-grade to get the French 155's that saw a lot of service in the campaign (mostly 155mm GPF).
SPMs- Steal them from the Marines. I would argue that their motivation should be Confident, but they did do some rather fearless things, so why not?
M3 Scouts- I don't remember seeing anything about them. The 26th did have M1 Scout Cars, but they are rather different vehicles. I don't think they ever formed units like this for Philippine service.
M3 Lights- Use them as is, but without the T30s and M4 81mm tracks. However they did have some of the Bren Carriers to use as scouts. Not a great replacement, but you take what you can get. I will point out that US Army doctrine, if you can call it that, was pretty… terrible. Worse still, USAFFE and its various commands were pretty horrible about armor use. Frequently someone would call for armor support only to be 'noped' by the 'experts' because of terrain, anti-tank weapons, etc. Also, frequently tanks could find themselves in the same area as other units, but not do anything to support them. The 26th had the wonderful experience of watching the tankers roll out and then immediately coming under Japanese tank attack. Yeah, with friends like that. They were generally used in penny packets of individual platoons or even just sections. Full company actions by the tanks was very rare. To be authentic you shouldn't field more than a platoon of the things for most actions, with the three tank 'platoon' not being uncommon.
Well there you go. Some notes on how to fool around with the US lists in The Pacific to get a more USAFFE in the Philippines style of army. Feel free to post your own ideas and tell this thread what you think.