Antenna connector placement
Let's suppose that the effective relative electric permittivity is above 3 (I don't have the dimensions of your stripline so I'll do some estimation). Anyway with an Er of 3, you'll have a wavelength of roughly 8 in, this is much much longer than the stripline length, so transmission line effects will not be noticeable from the short stripline. In general making transmission lines short will reduce the effects from them, and is actually a way I use to simplify designs (in some cases). 

You don't need to worry about noise from the Xbee SX 868 affecting the signal around the SMA as the SMA connector has ground bulit into the case and serves as a shield, the limited area that is exposed will provide minimal capacitance for coupling RF/noise. 

In many respects, moving the SMA closer to the source is better, make sure you leave the ground plane below the trace even if it is shorter. And I also like to run the trace around all four posts of the SMA on the top layer for the sake of betterness (even if it is minimal, might make it slightly harder to solder though).