I know you asked for contextual evidence and I hope to get there. However, when it comes to these sorts of things, contextual (which is part of internal) evidence is really only one of the factors that goes into these sorts of things. overview of internal and external evidence
The UBS (4th ed.) also has αὐτοὺς (which is unsurprising given the overlap between NA and UBS). The only other significant variant is ἡμᾶς which is the variant that you have noted in your question.
External Evidence
The UBS selected αὐτοὺς with an {A}
degree of confidence (the highest certainty). It is attested in uncials: א (Sinaiticus), A (Alexandrinus), miniscules: 209 1006 1611 1841 2050 2053 2344 2351, the Byzantine [046] text, the Wordsworth-White and Stuttgart Vulgates, Syriac Philoxenaina and Harclean, Bohairic Coptic, Armenian, Ethiopic, and some early church father attestation. Many of these date from the Fourth Century with the Coptics being from the Third Century.
ἡμᾶς is attested by some Old Latin manuscripts (Eighth and Ninth Centuries), the Clementine Vulgate (Fourth Century), the Sahidic Coptic (Third Century), and a few other early church fathers.
As far as external evidence is concerned, given the strength of Sinaiticus and Alexandrinus, and the geographical distribution of the witnesses, the committee favored αὐτοὺς.
Within the same passage, there is another variant in 5.9 that sees more diversity. UBS chose τῷ θεῷ ({A}
) over τῷ θεῷ ἡμᾶς, or ἡμᾶς τῷ θεῷ, or just ἡμᾶς. We would need to repeat the process for these variants as well. For the sake of length and time, I'll abbreviate the external here. The UBS-preferred reading is attested by Alexandrinus and the Ethiopic texts. ἡμᾶς is attested by a Vulgate manuscript and Greek lectionaries. τῷ θεῷ ἡμᾶς by Sinaiticus and most of the witnesses above for αὐτοὺς. ἡμᾶς τῷ θεῷ is not attested well but does have some witnesses in the Old Latin and Syriac.
Internal Evidence
You've noted the change in meaning wrought by addition of ἡμᾶς. It seems to be a scribal insertion to smooth the narrative, perhaps to make the πρεσβυτέρων (and ζῴων) the more direct recipients of this power. Or because it just makes better sense to use "we" in context because they're the ones actually talking.
βασιλεύω
5.10 also has a variant here (which you've noted). βασιλεύσουσιν ({A}
) is in Sinaiticus, St. Petersburg, and quite a few of the above witnesses for αὐτοὺς. βασιλεύουσιν in Alexandrinus and other minuscule found above. Finally βασιλεύσομεν is, once again, poorly-attested in only one minuscule, in the Clementine Vulgate, and in two early church fathers.
On the balance, it seems that external evidence was used to arrive at this decision. Because of the external evidence, ἡμᾶς ("we" language) was ruled out as original and the need to remain consistent within this section arose.
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So What?
The Textus Receptus is a text that is based on, follow me here, Erasmus' translation of the Latin (itself a translation from the Greek) back into Greek. Setting aside the margin for error in this, and setting aside many of the issues that have arisen from critical analysis of the TR, most of the Latin manuscripts contain this insertion because they were based on the Vulgate and other late Latin translations which have been demonstrated to have included this variant. TR includes this variant because of its textual heritage.