Implications of routing same signal on top and bottom of PCB?
What you propose to do is fine and normal.  IIC is limited to rather low frequencies, so a few mm trace length difference is inconsequential.

Your connector is apparently thru-hole, so the signals are available in all layers.  All you're doing is wiring a resistor to one of them.  The fact that there are other things wired to other layers of the same thru-hole pad (acting like a via in this case) doesn't matter here.

You are way over-thinking this.