Attenuation - Oscilloscope Front End
Your high end relay seems overkill to me if your bandwidth is only to 1MHz. Have a look at (shielded) reed relays. Will be cheaper too.  

1MHz is not a very high frequency, and neither is 1M\$\Omega\$ an extremely high value, so I'm a bit surprised to read that your Z is "all over the place". The JFET input buffer is a good idea. It will give you a very high input impedance; input offset current for a common TL081 is less than 200pA, so can control the impedance and scaling easily with resistor dividers.
Keep traces short and not too close to adjacent traces. For the input protection I would use low leakage diodes to clamp the input voltage between the rails.
If this doesn't help, explain in more detail what "all over the place" means, exactly.