Interesting points - and as you say I think the issues raised have more to do with what you see as managerial failings rather than favouritism.
Playing devil's advocate - the 'proven failures' that you are referring to are basically Luis ans Xhaka. But I'm not sure I would agree. They are both error prone - but certainly since Xmas they have played really well otherwise. Which is a dilemna for Arteta because 'trying something different' by starting the likes of Saliba or giving AMN a go is likely to result in their replacements making mistakes also - Saliba because he is so young and AMN plauying out of position. Indeed - as good as he has reportedly been for Nice, Saliba has made one or 2 howlers himself - which is totally to be expected. For a young manager - I can see some sense in him wanting to have experienced players on the pitch.
I'm not sure that we can really level the accusation of failing to try something different on a manager who has changed the way we play hugely from earlier in the season, and brought in/clearly developed Saka; EMS; Odegard and Cedric, plus switching up his central defenders on a regular basis?
What you and Ollie point to is the Pepe Willian dilemna. And of course I agree that this is a head scratcher on the face of it. Pepe in particular has done enough IMO both in terms of his performances and by comparison with Willian to merit starting over him. I disagree with what we have seen from the manager over this. But in terms of 'favouritism' I am willing to bet that 75% of fans will have a similar gripe with their managers. Not sure that Arteta is an outlier here. And he (not me) would argue that Wilian is our top assister this season, and has not been error prone or wasteful with the ball...