the "seudas mitzvah" that Eisav prepared

Last week I mentioned R' Sadya Gaon's chiddush that Yitzchak sent Eisav out to hunt game for him because אבל הבאת דבר מיגיעו ועמלו {של בנו} היתה לו לנחת רוח.  Yitzchak was not really a connoisseur.  You see that he could not tell the difference between fresh game meat and the goats that Rivka had Yaakov prepare.  What made the food that Eisav served special is the effort that Eisav put into getting it.  It's like when your kid brings home a work of "art" from school that you post on the wall or the fridge.  It's not the beauty of the picture that makes it special -- it's the effort of the child to make it which does.

On Shabbos I said over this RS"G and added that R' Tzadok haKohen in Pri Tzadik writes that what made Eisav's food special to Yitzchak is that it was infused with the mitzvah of kibud av v'eim, the one mitzvah that Eisav excelled in.  It was a seudas mitzvah -- the mitzvah of kibud av!  That's why, explains R' Tzadok, Rivka had to COMMAND Yaakov to listen to her and prepare the goats,  וְעַתָּ֥ה בְנִ֖י שְׁמַ֣ע בְּקֹלִ֑י לַאֲשֶׁ֥ר אֲנִ֖י מְצַוָּ֥ה אֹתָֽךְ (27:8)  Had she not done so, Yitzchak would have immediately known that it was not Eisav because the taam of kibud av would be missing.  By ordering Yaakov to listen to her, Rivka created an opportunity for Yaakov to fulfill kibud eim and infuse the goats with that same taam.  

2) Rashi explains that Yitzchak warned Eisav to go out to the forest to hunt and not bring him stolen food.  וצודה לי – ולא מן הגזל.  If Yitchak thought Eisav was a good guy, why would he need to warn him not to steal?

Maharal in Gur Aryeh answers:

מן ההפקר. ואף על גב שהיה עשו בחזקת כשרות, היה צריך להזהירו שלא יצוד חיות ועופות בסמוך לעיר, כההיא דתנן מרחיקין מן העיר, דלאו ודאי גזל הוא, והיה יצחק סבור שמא יעלה על דעתו שכבוד אב ואם דוחה אסור דרבנן, דהיינו ספק גזל - דדבריהם, ולפיכך הזהירו שגם זה לא יעשה, אלא ירחיק מן העיר.

I am not sure how to explain this hava amina (for whatever a hava amina is worth) that a mitzvah d'oraysa might be doche a safeik issur gezel derabbanan.  The safeik gezel is a dinei mamanos issue, so sfeika derabbanan l'kula does not come into play (and were it that simple you would not need dechiya).