I'm not contesting that an 18" saddle may be too long for the horse. Haven't see him, so I can't say one way or another, but I agree there's a good chance it won't.
However, to say that "femur length has little to do with seat size" is 99.8% incorrect. Perhaps in a perfect world where the a rider could have a saddle custom made for him/herself, that rider may be able to purchase a saddle custom made for his/her leg such that the seat size would be determined only by the size of the rider's own seat, but that is simply not the world that the vast majority of people reading this thread (nor the OP herself) are living in. For an off-the-rack saddle, particularly those saddles not available in a long flap option (as most options within the OP's price range will not have this option available), the length of the rider's leg from hip to knee is absolutely going to dictate, to some degree, what size saddle s/he can ride in. Also, though dressage is certianly ridden with a longer stirrup than jumping, a rider new to dressage (and really, the vast majority of all dressage riders below FEI levels) are going to ride with some bend to their knee. I suppose one could say, "Well, my butt fits in X size saddle, so I'm going to buy one that size regardless of where my leg falls on the flap or how hard I may have to fight against the saddle to maintain a proper position!" Or, in the alternative, one could accept that taller riders do, quite frequently, have to ride in seat sizes larger than their rear ends may otherwise dictate, so as to have their leg fall appropriately on the flap to be supported rather than hindered.
No idea why you feel the need to be so combative about this, but it simply is not correct to say or imply that only fatties need a saddle larger than 17.5".
And regardless of your snide comment, "close contact" is a perfectly acceptable term for a jumping saddle.