I have left my amateurish writing and acceptance of traditions below in favor of quoting a number of other scholars who have actually done extensive research on this...
I have linked two papers here that address these variants at length, and brought their conclusions below. Both of them conclude that the MT is the authentic original here.
(One strong point found in both articles is that in SP and LXX, the order of Canaan and Egypt is reversed, and that the word ארץ is not present in the SP.)
Resolution of Exodus 12:40 Textual Variant, Douglas N. Petrovich
The Israelite Sojourn in Egypt: 430 or 215 Years? A Text Critical Analysis, Wayne A. Mitchell
Mitchell looked at the 7 (!) variants of this verse, and concluded the following (Variant 1 was the MT):
In this investigation of the manuscripts of Exodus 12:40, the variants were examined with the tools of text criticism, which included a search for haplography in the two ancient Hebrew scripts. Two of the variants were found to have possible support for a single-letter haplography. If this information was the only text critical observation concerning all the variants, the evidence would be significant.
However, the following additional observations of the variants must be considered. Of the variants studied, the longer readings were found to have the additional words 1) in different locations (“they and their fathers”), 2) in different word orders (“Egypt and in the land of Canaan”, “Canaan and in the land of Egypt”), and 3) with different wording (“them and their fathers”, “and their fathers”, “in Egypt”, “in the land of Egypt”). These facts are very convincing evidence of a secondary origin of the longer readings, i.e., scribal expansions. In this case, “lectio brevior praeferenda est,” the shorter reading is preferable. For these reasons, text critics have rejected the longer readings found in variants 2-7, and have concluded that the MT reading is likely the original.
Petrovich's conclusion (Variant 2 was the MT):
Both external and internal evidence convincingly point to Variant 2, the reading of “in Egypt,” as the original text found in Exodus 12:40. The reading in the LXX, the Samaritan Pentateuch, and Josephus — although he offers conflicting positions on the issue — cannot supplant the reading of the MT and the DSS, as the evidence for Variant 1 is forced, leading to a non-contextual interpretation and an indefensible position. The textual and historical data
related to the internal evidence clearly makes a reading of “in the land of Egypt and in the land of Canaan” (Variant 1) impossible to sustain with credibility. Variant 2 also suits the context of Exodus 12:40 far more naturally, as the entire story — from the book’s outset — deals solely with
the nation’s stay in Egypt, not with the patriarchs’ sojourn in Canaan before Jacob departed for Egypt. Choose Variant 2 with great confidence, giving preference to the reading found in the MT: “in Egypt.”
What is most interesting about this situation is that this is one of the (few) changes that the Talmud notes was made in the LXX. Megillah 9a states (Sefaria text and elucidation):
(שמות ד, כ) ומושב בני ישראל אשר ישבו במצרים ובשאר ארצות ארבע מאות שנה (שמות יב, מ) וישלח את זאטוטי בני ישראל (שמות כד, ה) ואל זאטוטי בני ישראל לא שלח ידו (שמות כד, יא)
Instead of: “And the residence of the children of Israel, who resided in Egypt, was four hundred and thirty years” (Exodus 12:40), which when read literally is imprecise, for they did not dwell in Egypt that long, they wrote: And the residence of the children of Israel, who resided in Egypt and in other lands, was four hundred years. Instead of: “And he sent the youth of the children of Israel, who brought burnt-offerings” (Exodus 24:5), which evokes the question of why young men were sent to perform that service, they wrote: And he sent the elect [za’atutei] of the children of Israel. The same term was substituted again several verses later, rendering the verse: “And upon the nobles of the children of Israel He laid not His hand” (Exodus 24:11), as: And upon the elect of the children of Israel He laid not His hand.
Based on this early report of this being an interpretation that was deliberately added into the LXX, I think that we can safely assume that the Masoretic text is correct here. It is quite well-known that the Samaritan text generally emends away difficulties, and that the later two sources are simply elaborations that contain the traditional explanation of how the Biblical Narrative gets the number 430.
This is noted by Luzzatto (among many other commentaries), who also gives his list of some authorities that clearly rejected any possibility of this being the original text. (I have tried to identify them, although I will not look through old latin commentaries to find where they say this):
Some other considerations are of course the accuracy of the number 430, as well as the context of this verse (see next verse which makes no such mention), etc, but I can trust the sources that we have noted above.