So Chris Young (91 PA: 205/319/295 and 66 wRC+ vs LHP) is our 4th outfielder/RH pinch-hitter and DH vs LHSP whilst Rajai Davis (116 PA: 255/319/396 and 92 wRC+ vs LHP) isn't? Still don't get how they complement each other - seems to me that Davis can do whatever Young can do a little bit better plus he has plus speed to boot.
(He may compliment him because I hear he's a nice guy but I'd say he supersedes him rather than complements him on the team)
I think you misunderstand what seems to be quite a likely Farrell strategy, regarding the 4th vs 5th OF debate once JBJ returns.
There are two starters who Farrell has shown willingness to platoon against LHP this season: Moreland and Benintendi. I expect that Farrell would be willing to PH for either of them in the future, since he's done so already. Farrell has not platooned JBJ at any point this season, at least to my recollection; Holt, I believe, will be a non-factor going forward since his main role will likely be to backup MI to give Bogaerts and Nunez a day off. So I don't find it reasonable to think either of those guys will see a PH replacement in the late innings.
Along with this assumption, it's important to note that Farrell is unlikely to bat a PH for any of the RHH except Vazquez, and only when he's hitting against an RHP in the late innings or extras.
Benintendi and Moreland are not generally placed in the lineup such that a single LOOGY is likely to be used against both of them in the same inning. Moreland is more likely to have a LOOGY brought in first, since he has generally been followed in the lineup by C and CF, while Benintendi is generally followed by Mookie and Hanley.
Therefore, having Young brought in to hit for Moreland first makes sense, because if he's going to be replaced in the field by another IF, you're not going to want to burn Davis's overall better skill set on one PH appearance where the expectation of marginal benefit is based entirely on SSS.
Of course, all of this assumes JBJ returns from his thumb injury this season. Until that should happen, Davis is the everyday CF and Young is the 4th OF.