Briony G works for the DM now I think (used to be the Telegraph) and she and Harry go back a few years - they were on a podcast about mental health together. It's going to be an opinion/personal piece rather than editorial or royal rota. I don't think the DM care about sides as such, as long as there's a juicy story involved.
But I agree, there's some PR strategy at play here about Harry. It just doesn't make a whole lot of sense though. One minute he's apparently missing home and seeking to curry favour with his UK relatives, the next he's calling Oprah his fairy godmother and promoting Spare in a California bookshop.
It may be that he's taken over his own PR after the departure of Josh Kettler, and that's why we are getting the mixed messaging. The People slip up in the article the other day (about the best gift he ever had being his kids), where they gave it away that Harry had emailed them rather than a spokesperson, could be a sign of amateurish dabbling and conflict at home with the messaging going out.
I don't know if anyone watches a YouTuber called River? He seems to be quite well connected and has some celebrities as fans, in the States in particular. Anyway, his gossip - obviously to be taken with the pinch of salt required to season all online gossip - from someone who works for the Sussexes is that Harry's reaction to Catherine's videos was a lot of sobbing. Whether that's because he wishes he was frolicking in the forest and reminiscing about the Fab Three that once was, or because he wishes his PR strategy was that good, is unknown.