Suerte, I do sympathise with you, it does sound like a big move for your DS, country change and school change. I just want to echo a few comments already passed. My DS joined at 10+ as well, and we have no regrets. I have some DN in the school and 2 joined at 11+ and 1 at 10+ ( last one), and their mum tells me if she could do it again she would've let them all go at 10+. A number of people I know who joined at 11+ said ( the parents), the wish they had joined at 10+, but some were not aware this was a possibility even. Obviously it is not for everyone to join that early, but the boys seem to settle quicker. I do believe the LG trip also aids this. the boys get to bond and also get to know their teachers.
The lower school is generally kept separate, the older boys have to ask permission and have a good reason to visit the lower school. My DS doesn't see his cousins until the end of the day usually, if that even. As someone mentioned it is easier to gain admission at 10+/11+ than other years ( obviously not impossible). And he will have to have sat a pre test in most 13+ schools. I know W do a pre test as well but I think it is for the only for boarders, at this stage.
About not displaying good sportsmanship, I think I know the thread you refer to, and I believe the person who wrote that thread ( although in a very responsible/ important role), clearly had her facts wrong on a number of things she wrote. I have been to a few W against DC matches and I have not experienced W boys displaying untoward behaviour across hockey, cricket, & rugby matches. I also only witnessed one match where the DC boys ( at 10+), were not very well behaved, using bad language, and passing racist remarks and also refusing to shake hands, with the W boys. But that was one match and I believe there were 3 boys only involved. For the record, I am in no way slagging DC boys, it just what I saw on the day. Other than that I've not seen bad sportsmanship displayed. If I remember correctly, also, on the thread mentioned, there was a DC parent who was 'slagging off' W boys, so it is not correct to say they don't.
There are a number of W boys who have siblings in DC, and I know some DC boys as well, and I don't think they are different to the W boys on the whole. But this is looking through my own little corner of life. I do agree that DC has a 'posher' feel to it. But horses for courses.