Firstly, another scholarly resource that confirms this is R. J. Ẕevi Werblowsky in The Character of R. Joseph Qaro's "Maggid" /לדמותו של המגיד של ר' יוסף קארו He states (fn 1, my rough translation) that all of the suspicions and claims that have been heard are null and void. (He also notes another book written on this topic, read the review here if you dare.) His opinion is significant, as he is the author of Joseph Karo : lawyer and mystic, where he tackles this issue at length. A short excerpt from the section dealing with this issue is available here, where various proofs are presented in favor of M.M.'s authenticity. Werblowsky's book is reviewed here by Joseph Dan.
Secondly, I'm not sure if this is exactly what you are looking for but in Shadal's Vikuach at Chochmat Hakkabalah he devotes a chapter to the Maggid Meisharim. Translations and notes are found here on Parshablog (see part iv and previous 8 posts). In part i he begins by quoting the Sefer Emunas Chachamim, Ch. 27 (which is also quoted by Chida in Shem Hagedolim), which brings some of the proofs suggested by Marvin J. Heller in Yishai's answer. Shadal considers the argument based on Sefer HaPardes (a sefer written by R' Moshe Kordovero in the lifetime of Rav Yosef Karo) to be a straightforward proof that the M.M. could not have been a forgery. Josh Waxman (translator there) provides Mevaqesh's counter-argument that perhaps "someone took that genuine quote and included it in a forged Maggid Mesharim to bolster its authenticity...". However, see the full quote from Emunas Chachamim, as this bolsters the argument, by noting the reputation of the Sefer, as well as the likelihood that it was read by Rav Karo:
וא"כ איך אפשר שדברים כאלה המדברים בסתרי התורה יכתוב הר"ם קורדואירו בשם רבו מהרי"ק בחייו ובעירו של מהרי"ק ויעלם על ספר ויפרסם אותם בפני רבו וישלחם להרמ"ע, ובכמה מקומות אחרים אם לא היה האמת שהרב קארו כתבם ופרסמם יתעקש המתעקש ויאמר מה שירצה שזו היא בעיני תשובה ניצחת שאין להשיב עליה והראיתי דבר אלה לאחד מן המתפלספים ואמר לי שצריך לומר שהרב קארו בדה דברי שקר כדי לגדל מעלתו, תאלמנה שפתי שקר אתפרכן אתחרשן הדוברות על צדיק עת
Shadal there assumes that if we would not consider Rav Yosef Karo to be a "completely wicked individual, like Yeravam and his colleagues" and as a result, it is impossible to believe that "he invented these words from his heart". (Note that later in the Vikuach, there are some passages that are pointed out that would be hard to accept as having been said by an angel, or accepted by Rav Karo. See here.)
The above, however, is simply a review of the introduction of this section. Shadal's main thesis is that Maggid Meisharim was never intended to be literally taken as a discussion between Rav Karo and an angel, but rather it was just him going through his thoughts in an imaginary dialogue. His proofs for this are as follows:
Hinted to in this statement:
(Venice printing, page 43c, Zalkwa printing page 50a {?} ): "Is it not so, that even though at times I explain some explanation on a verse, which is not so, am I not saying according to that which in your will."
various statements noted in parts iv - viii of parshablog links
The statement of the Maggid (Venice, 47a, Zalkwa, 51b):
"It is I who speaks with you, your soul {neshama}, not the nefesh nor the ruach, but rather the neshama herself. And is it not so, if prophesy has ceased from Israel, from you it has not ceased, for each time I come to you to direct you in which way to go."
hinted to in the introduction of Rav Shlomo Alkebetz which was written in the beginning of the Lublin printing
- Whenever the (aformentioned) Sefer Hapardes quotes the Maggid, he refers to it as "My teacher Rav Yosef Karo", as opposed to "the angel" or something like that.
These proofs are hammered out in the parshablog posts above, I would suggest that anyone interested read them directly, as well as various other related parts of Shadal's Vikuach.
This is apparently also the view of the Chazon Ish (here, Section 6, h/t to DoubleAA), and R' Jose Faur (as per Mevaqesh).