We can answer as follows: If you examine the issue carefully you will see that Geirus brings the Ger into 3 new and different areas.
1] He is part of the Chosen People and not a member of another nation like he was before. So he is both part of the Jewish Family and also attains Kedushas Yisrael. [Rav Naftoali Trop most famously taught (כתובות י"א) that there is a possible scenario where a Ger would be part of the Jewish family while still lacking Kedushas Yisrael].
2] He has all the rights and privileges of Jews including the Halachic "Law of Return" allowing him to live in Eretz Yisrael. He don't have an individual, ancestral portion but still has a general right to live there. In the future, he will even have their own portion, as the psukim in Yechezkel teach [47/21-23. See Mishna La-melech Bikkurim 4-3].
3] He may now learn Torah as opposed to גויים who may not.
Now the pasuk reads beautifully:
"וכי יגור אתך" - He will live WITH YOU. He is a member in good standing of our Jewish family.
"בארצכם" - He now has a right to live in your Land.
We also understand why the pasuk opened with לשון יחיד and says "אתך" and concluded with לשון רבים saying "בארצכם". He is indeed a personal member of the family of every Jew. אתך. But when it comes to the Land he lacks a personal portion but just has a GENERAL portion in the Land. בארצכם. Plural. Together with everybody else.
Now we can understand what compelled Rashi to explain the pasuk as he did. When it says "אתך" and "בארצכם" it is not telling us a place or time but defining the acceptance of the Ger in our midst. We are accepting him אתך - as part of our family and בארצכם - as one who has rights to the land. [That answers the Alshiach;'s question why it had to say "אתך" - to show that he received from us the right to be part of our family]. On this reality the Torah warns us - לא תונו אותו. Don't vex him and cause him pain. How would one do that? Well the pasuk already alludes to two of the aspects of Geirus so all that is left is the third - Talmud Torah. He may now learn like a full fledged Jew. So Rashi HAD to explain that the אונאה referred to by the Toras Kohanim is about Talmud Torah, that one shouldn't say to a Ger that yesterday he was serving idols and today he is coming to learn Torah??! So it was פשוטו של מקרא that compelled Rashi to explain as he did.
This will clarify the two variant explanations of Rashi. In Behar the issue is to cause the Ger pain in a more general way. Telling him that the mouth that ate treif is now learning Torah?! Or telling him [as the Gemara says] to remember his earlier deeds. "Remember when you smoked weed in college?" "Remember you non-Jewish girlfriend?" And if he is suffering telling him "hey dude, you have a LOT of Aveiros for which to atone". Those are all more generic forms of emotional torment.
Here we are talking about "פסלing the Gavra", telling him that he is not worthy of the rights accrued by born Jews. The pasuk alluded to his rights to be a member in good standing of our Jewish family and to live in Eretz Yisrael tipping us off to the fact that the torment is that he is not fully worthy of the right of born Jews which could only be Talmud Torah [because that is the only right not mentioned by the pasuk. So the other rights he has "אתך" "בארצכם" and we are telling him that the right to learn he doesn't have.] So we don't mention the mouth, the כלי of Talmud Torah as being unworthy [as in Behar], but the very fact that he is learning at all.
