I've watched both Lankshear and Dorrington for the youths several times. They are both good, promising players but neither are yet ready for the PL. Both probably need a couple seasons out on loan to lower level (Div 1 or champtionship) teams. You are correct that they probably should've gone out, though I think there were extenuating circumstances for both.
For Lankshear, I suspect that as well as Ange knowing that we were trying to punt Richarlison on, as well as him constantly being injured, that means we'd only have Solanke as a number 9 option. I suspect it was also felt that Lankshear would still get something out of playing age group football (which I agree with). I suspect Veliz wasn't retained because of Europa league squad position would've meant he was only in the PL squad. You already know my opinion on us leaving ourselves very short for our PL squad and I would've happily retained Veliz for that but maybe, again, Ange is looking long term and feeling Veliz playing lots of games at the top level in Spain is better for his and our future than him getting a few bits and pieces of minutes in the premier league?
For Dorrington, I think he was injured at the start of the season so perhaps not really likely to get a loan. With Phillips, again, I think the manager is thinking long term and considering the fact that Phillips playing every game for a club in the Championship was better than him sitting around at Spurs with 4 central defenders ahead of him in the pecking order and therefore only playing age-group football that he has now clearly outgrown. Of course this looks like a bad decision now but that's because we've had a bit of a freak run of injuries in that position.