Mendy played as a wingback in a 3-5-2 before he tore his ACL. Up the pitch as you described.
End of the season he came in as a LB like for like in a back 4 for 15 mins each, playing much deeper. Both is possible.
If he plays 30 yards further up in a back 4 we will have to cover that huge hole as a team, a DM alone is not able to.
Pep will be aware I guess.
Sorry but you seem to be quite deliberately missing the point.
You simply cannot compare the defensive help in midfield offered by Delph and Mendy tracking back, it's blindingly obvious there's a major difference. Also in each of Mendy's cameo appearances at the end of last season he got caught up the pitch - that's the kind of left back he is and - you can't be hitting the byline to cross and be offering defensive cover in midfield at the same time.