Islam an Arab-only religion? That's very untrue and proselytization was at the heart of this religion from the very beginning.
That's caricaturizing the point, that was less Arab-only religion in the early centuries, than Arab-first.
1) You had an Arab cultural dominance, with Arab being the religious but as well political and cultural language; eventually, it comes to degree of arabisation being one of the main indication of social advancement.
The Pact of Umar is really interesting on that matter, even if its secondary authenticity (hard to say if the whole was concieved later or if it was a genuine account written down later) is debatable (as it was arguably considered genuine by later Islamic scholars, and used this way), especially the parts where Christians were forbidden to speak Arab, to read Qu'ran, to not use Arabic script, etc.
While it was diversly applied, critically after the VIIth century, it shows a will to make Arab and Islam tied up.
2) Proselytization was limited in the two first centuries of Islam , safe a wave of conversion in the first conquests of the concerned regions, but conversions didn't made it to equality of treatment.
Converted peoples as Berbers were considered as second class Muslims, being forced to pay the taxes raised by Byzantines, (monetary, or physically, such as sending troops to whoever ruled in Carthage) and to pay jizya as well, with general harassment.
Blaming it all on Umayyads is really pushing it : their indications on that matter (or other ones) were often ignored if being in opposition to local Arab nobility wishes.
The whole Arabic cultural hegemony, and refusal of indigenous Islam (for more just than Arab dominance, political centralisation of political/religious policies on Caliphe's head played a role, critically when they were all Arabs up to the Fatimids, a rival and heterodoxial dynasty)
Muhammad's policy is hardly representative of these situation and provinces conquered during the Rashidûn era remained largely Christian up to the XIth century (and having large countryside Christians populations up to much much later).
And classical Arabic is a highly flexible language where one word can have multiple meanings. That's why some translations say slightly different things.
That's the case for a lot of languages, and not always went against regular or official translations. Classical Arab kept growing more hard to translate popularly partially thanks to its "sanctuarisation".
Nothing really prevented a strong caliph/emir/etc. to gather around him enough legitimacy and scholars to undergo such official translation, safe as you said, the heresiac risk but that could be turned around, especially giving the religious caliphal authority; and the feeling that Arab should remain the language of Qu'ran, at least officialy (and therefore in mainstream religious usages).