Avi, unitarians make up .3% of all christians in the USA. They are halachiklly a miuta dimiuta if even that much. The overwhelming majority believe in the trinity; the poskim say this is shituf. There sre deos yechidim that for goyim shituf is not assur, but for yidden it definitely is and we are forbidden to support it in any shape or form.
As for veterans vis a vis hakaras hatov; we don’t find a source in chazal as far as I know that interprets hakaras hatov that way. We have the gemara quoted above, which attributes tzedaka to nochrim as darchei sholom, but not an ends to itself, and as such if a person gives without the understanding that he is a frum jew, it will probably be assur the same way hashovas aveidah is assur; additionally it is a very serious issue of lo sichanem, giving free items to nochrim (or even compliments) is an issur/prohibition.