To make the comparison a little easier, here are the texts (with key below):
- Stephanus (1550) Textus Receptus = TR
- 28th edition Nestle Aland = NA28
TR: Καὶ ἤρξαντό τινες ἐμπτύειν αὐτῷ
NA28: Καὶ ἤρξαντό τινες ἐμπτύειν αὐτῷ
TR: καὶ περικαλύπτειν τὸ πρόσωπον
NA28: καὶ περικαλύπτειν αὐτοῦ τὸ πρόσωπον
TR: καὶ κολαφίζειν αὐτὸν καὶ λέγειν αὐτῷ
NA28: καὶ κολαφίζειν αὐτὸν καὶ λέγειν αὐτῷ·
TR: Προφήτευσον καὶ οἱ ὑπηρέται ῥαπίσμασιν αὐτὸν ἔβαλλον
NA28: προφήτευσον, καὶ οἱ ὑπηρέται ῥαπίσμασιν αὐτὸν ἔλαβον.
Bruce Metzger, in the second edition (1994) of A Textual Commentary on the Greek New Testament, gives an "A" grade (certainty) for the UBS4 placement of αὐτοῦ (which is the same as the NA28 for Mark 14:65). He argues that readings with the alternate placement were influenced by Matthew 26:67. So "καὶ περικαλύπτειν αὐτοῦ τὸ πρόσωπον" is the preferred reading for this clause.
The final difference is most likely a transposition error made by a scribe (ἔβαλλον rather than ἔλαβον). The transposition causes little change in the overall meaning in this case (they "received him with blows" vs. "they struck him with blows"). However, there are some scholars who see this as indicative of a Latin idiom (verberibus eum acceperunt) translated literally into Greek (cf. Joel L. Watts, Mimetic Criticism and the Gospel of Mark, 2013, p. 76).
There is also a longer reading of the final clause. According to Metzger,
The longer reading involving the addition of the question τίς ἐστιν ὁ
παίσας σε; (“Who is it that has struck you?”), with or without the
introductory Χριστέ, appears to be an assimilation to the text of
Matthew (26:68) or Luke (22:64). The shortest reading, προφήτευσον,
supported as it is by the Alexandrian text and several early versions,
best accounts for the rise of the other readings.
Since neither the TR reading cited in the question nor the NA28 contain this reading, I will not address it any further.
Also, to directly answer your original questions:
What is the conjugation of each verb, and what is its meaning?
- ἔβαλλον = 3rd person plural imperfect active indicative = "they were throwing / striking"
- ἔλαβον = 3rd person plural aorist active indicative = "they received / struck"
How can one explain the difference between the two textual variants?
- Concerning the placement of αὐτοῦ, it is likely that scribes attempted to make the text line up better with Matthew 26:67.
- In regards to ἔβαλλον vs. ἔλαβον, this is likely a simple transposition error.
What is the earliest (estimated) witness in support of each variant?
- Metzger finds the earliest support for the UBS4/NA28 reading concerning the placement of αὐτοῦ, with extant manuscripts dating back to at least the 5th century.
- The reading ἔβαλλον is supported pretty much solely in the Majority (Byzantine) text-type while virtually all other text-types support the reading ἔλαβον, making the latter the preferred reading.
So in summary, these are likely scribal transpositions. One changing word order, probably to conform better to a parallel account, and the other a simple transposition of letters within a single word, likely a copyist error passed down in the Byzantine textual tradition.