שלום וברכה!
[כ' סיון תשכ"א]
... במענה לשאלתו: יכול להתפלל בהברה ספרדית או אשכנזית, אבל לא לערב ז.
א. שצ"ל כל התפלה בהברה אחת.
נדפסה בלקו"ש חי"ט ע' 463.
*) הנ"ל הוא טעם על שבקשו מלכתחילה ג' ימים. - שקר, כשאלתו, לא הי' בזה, שהרי לא דברו בכלל מה יהי' לאחר ג' ימים, ופשיטא שלא אמרו שלאחר ג' ימים נשוב (וכדיוק פי' הריב"א עה"ת ר"פ בשלח), אלא - שזה יהי' תלוי ויתברר אז - ורק עפ"ז מובן, מה שביצי"מ ראו המצרים (ולא שאלו ע"ז) שלקחו בנ"י עמם כל נכסיהם לא השאירו דבר.
וק"ל.
Shalom u'Vrachah!
In response to your question: One may pray using either Sephardic or Ashkenazic pronunciation, but should not mix them. That is, the entire prayer should be recited in one consistent pronunciation.
This was printed in Likkutei Sichos volume 19, page 463.
The above is the reason why they initially requested three days. Regarding your question—there was no deception here, as they did not discuss at all what would happen after the three days. It is obvious they did not say that after three days they would return (as explained precisely by the Riva on the Torah at the beginning of Parshas Beshalach). Rather, it depended on what would be clarified at that time. Only with this understanding does it make sense why, during the Exodus from Egypt, the Egyptians saw (and did not question) that Bnei Yisrael took all their possessions and left nothing behind. And this is clear.
Summary
The Rebbe teaches that prayer should be recited consistently in one pronunciation and explains that the Israelites' request for a three-day journey in Egypt was sincere, with future intentions left open—not deceptive.