More than just leaving it as them believing in shituf, the Rebbe pointed out a number of times that by now that we can assume that the vast majority of christians actually believe in achdus Hashem, and at most pay lip-service to ideas such as the trinity (sicha of Shabbos parshas Shemini 5743, maana to Rabbi Krinsky 28 Sivan 5749, and elsewhere).

On a side note, it is very disappointing that there is still a need to defend the very things which the Rebbe already discussed at length numerous times, in public and in letters, throughout the years (e.g. regarding non-denominational prayers, governmental support for religious private schools, moment of silence, the theocratical foundations for the USA, support for sheva mitzvos, etc.) and whereas the Rebbe was addressing rabbonim from outside Chabad who were complaining, now it is Lubavichers themselves making these taanos.

The Rebbe on multiple occasions praised the fact that American currency has on it “in G-D we trust”

legufo shel inyan, the Rebbe constantly quotes the shitos (a haara of the Rebbe with a collection of the shitos on this issue is in mayim rabim 5717, printed in melukot) that shituf is mutar for bnei noach (shituf means that Hashem is the Big Boss, but CH”v he has some partners or helpers). the vast majority of bnei noach in the us (if they even believe in that religion) believe in the manner of shituf, not avoda zarah mamash.
the Rebbe constantly spoke about the emunah of the founding fathers in Hashem, and what it says on the money of the us, advocated (at various points in time) for a nondenominational prayer in schools addressed to G-d

so in short, i think that it’s incorrect to say that he’s praising a”z ch”v, to the contrary, i think this is a beautiful expression of his emunah baHashem, and should be lauded and encouraged in the nation at large

You cannot have Hanaah from the money. You can consider me the “YAM” and throw it in me.

Which says on them in g-d (which according to you is ע״ז) we trust.