Old Palace was not equally academically selective, not by a long shot 
It wasn't when I chose Croydon High over Old Palace almost 40 years ago, and it certainly wasn't years later when they took over Croham Hurst when it closed and accepted all the far less academic girls from there.
Its reputation never really recovered from that, and together with the lack of grounds and the central Croydon location putting off many parents, it had been running at way less than capacity for years. All the trinity and whitfgift parents I know refused to send their daughters to Old Palace, most choosing Croydon High or Caterham instead. I am surprised that parents with daughters there hadn't realised that it might be in trouble, you only had to look at the pupil numbers.
Bursary funds are built up partly via parental contributions. If a charity asks for donations for a specific reason, they are not able to divert funds to another project. So if people have donated to the Trinity bursary fund, then the Whitgift Foundation is not allowed to then use those monies to subsidise Old Palace. So the fact that Trinity is providing 100% bursaries is not remotely relevant to the closure of Old Palace.
There has been a long history of indie girls' schools closing in the Croydon area due to lack of demand: just in my living memory I can think of Croham Hurst, Commonweal Lodge, and now Old Palace, and there are others near(ish)by that have had to go co-ed, such as Dunottar. But sure, it's all down to the Whitgift Foundation.
But anyway. OP, your son has done amazingly well, congratulations to him. I am sure he will thrive at any of the schools!
My DS were/are at Trinity and I would echo PPs in saying that it's a great school - academically, pastorally, musically, sport - and very down to earth. Equally, I have friends with DS who are very happy at Dulwich.
I would definitely consider your journey time though. It's not just about the time for your DS to get to and from school - don't forget about parents' evenings, after-school meetings, and concerts, and it's also good to have school friends nearby rather than 15 miles away!