Ok guys here's my take on it.
Bearing in mind GNE took on most of the Peterlee network commercially in October (probably because they lost the tender... to Scarlet Band no doubt!), these changes were always going to come.

The 202 is going to Seaham only from Peterlee, removing the Sunderland link there. This straight away gives the X7 an advantage. This is just my thinking and got no fact behind it... but, why number Middlesbrough-Sunderland express X7 and not X8? My thinking is that once the X7 has killed off the top end of X35 (Peterlee-Sunderland) they'll re-route the X7 to cover parts of it, while the Hartlepool-Peterlee part of X35 becomes X8 and gets extended to Newcastle... thereby giving a Hartlepool-Newcastle express with a ready-made customer base to expand from. This would give an X7/8/9/10 numbering too.
It looks as though the X7 split will be down to tacho reasons, to make it "cheaper" and easier to operate from the word go.

As for the 65, becoming 265... is there a reason the dual-door B10BLEs at Percy Main haven't been branded despite being yellow? 265 would of course be inside the East Durham brand numbering system... whereas Percy Main would take on some of the ex Crusader BLEs to replace the yellow ones. The W-reg ones would presumably be used on the X7?