A "scholar", well I don't hold scholars any different than anyone else.
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Generally, they are more informed, their views based more on evidence and less on opinion.
Certainly might mean that they have read and studied, but it doesn't mean that they know the truth.
Doesn't mean they don't, doesn't mean non-scholars do ...
When I read words like these, they contain no factual evidence. I might have gone to the moon. No one knows except for me.
Just applying that rule to your statement about Mark – we don't know.
Why do Matthew and Luke quote Mark verbatim? Probably because they were copying his texts. Luke states at the outset he is drawing his gospel from sources. Most likely Matthew was as well. In Matthew and Luke's case, we know a source was Mark.
With Mark it's trickier – some argue he drew from an early Matthew – others from Peter – only they know.