I have great scholarly affection and respect for Prof. Beale (I'm using a number of his books presently), and this is his explanation of the Mark 1:2 variant, which reads "Isaiah the prophet" per the Nestle-Aland/United Bible Society (NU) text, and it's a pretty good explanation. However, the TR and MT read simply "in the prophets", so there is no apparent discrepancy to begin with. Dr. Beale is a CT guy and I take that into account when I use his masterly work.
There are many excellent books which treat of the topic of apparent discrepancies, and are most useful to the Bible student. In fact, they are necessary when dealing with a person like Bart Ehrman who seeks to use such phenomena as a weapon against the inspiration, infallibility, and preservation of God's word. I have collected every one I could find, for they are very useful.