Did the malachim eat meat and milk together?

First, see the possuk with Rashi:

8. And he took cream and milk and the calf that he had prepared, and he placed [them] before them, and he was standing over them under the tree, and they ate.		ח. וַיִּקַּח חֶמְאָה וְחָלָב וּבֶן הַבָּקָר אֲשֶׁר עָשָׂה וַיִּתֵּן לִפְנֵיהֶם וְהוּא עֹמֵד עֲלֵיהֶם תַּחַת הָעֵץ וַיֹּאכֵלוּ:
And he took cream and milk, etc.: But he did not bring bread, because Sarah became menstruous, for the manner of the women returned to her on that day, and the dough became ritually unclean. — [from B.M. 87a]		ויקח חמאה וגו': ולחם לא הביא לפי שפירסה שרה נדה, שחזר לה אורח כנשים אותו היום, ונטמאת העיסה:
cream: the fat of the milk that is skimmed off the top.		חמאה: שומן החלב שקולטין מעל פניו:
and the calf that he had prepared: that he had prepared. Each one that he prepared, he took and brought before them. — [from B.M. 86b]		ובן הבקר אשר עשה: אשר תקן, קמא קמא שתקן, אמטי ואייתי קמייהו:
and they ate: They appeared to be eating. — [from here we learn that a person should not deviate from custom. — [from B.M. ad loc., Gen. Rabbah 48:14, Targum Jonathan]		ויאכלו: נראו כמו שאכלו, מכאן שלא ישנה אדם מן המנהג:

From the possuk it clearly seems that the malachim ate the tongue (according to Rashi) together with the butter and milk.

While I'm unsure whether eating meat and milk is an issur deoraisa, (IIRC its only cooking or benefiting from the cooked mix, but I might be wrong, since there is a third limud that I can't remember,) I'll assume that an issur derabanan would be enough here.

Now to the central issue:

Sifsei Chachamim asks how this came to be that Avraham gave the malachim basar vechalav. And he answers that he gave them the dairy before the meat (and they waited a while in between). Alternatively, he says that Avraham possibly held like the opinion that washing in between is enough.

I have a few issue with this pshat.

• As Rashi clearly states above, the Malachim didn't really eat; they only appeared so. Tosafos, does hold that they really ate, though.
• Avraham assumed that they were Arabs, he definitely didn't assume them to be Jews since there weren't any! So what would be his issue with serving this to non-Jews?

Now there is some basis in Rashi for such a pshat. As Rashi says above: "And he took cream and milk, etc.: But he did not bring bread, because Sarah became menstruous, for the manner of the women returned to her on that day, and the dough became ritually unclean." According to what I wrote above, there should be no issue for non-Jews with ritually unclean bread, so whats Rashi's problem.

It therefore seems according to Rashi, that Rashi held that Avraham was going to eat along with them, thereby prompting the Sifsei Chachamim's question above. (In Avrahams case, at least.)

However, on this very Rashi on Sarah, the Sifsei Chachamim has a different problem which he resolves by saying that Avraham couldn't serve the bread because it was chometz, and it was Pesach at that time.

Chometz on Pesach cannot be served mide'oraisah at all.

In conclusion, it seems that the Sifsei Chachamims problem is a non-issue, unless he has other evidence of Avraham joining the meal. (There is ample evidence for this according to other Rishonim. He doesn't quote them here, however.)