I am wondering about this combination. Stalwart requires DieHard, Endurance and likely Combat Expertise. Giving ultimately a DR10. I think these feats:Stalwart and Improved Stalwart are a means to improve the Combat Expertise feat. The application is still awkward as the DR is capped at level the level 16 CE level....(not here or there however , just an observation! Maybe level 20 Combat Expertise and Improved Stalwart should give a 10% Fortification????)
However , back to point why not let Shield of Swings give access to Stalwart and it's improved form. Mechanically interesting as you still suffer loss of 50% damage and the DR would only go to a maximum of 8 (based on SoS AC bonus of 4). Was this even considered by the developers? It doesnt seem unbalanced to me, Shield of Swings like Combat Expertise is not a "must have" feat. The Feat Tree concept is nice but allows for lack of integration of all the rules books, why not look for interactions and allow them where thematic? So the Feat "Bush" for Stalwart and Improved Stalwart would be Diehard,Endurance and either Combat Expertise or Power Attack/Shield of Swings leading to very similar effects but different flavours!