The phantom power should (or must) be conected to the chasis, since pin 1 of the XLR MUST be connected to the chasis directly and is where the mic takes it's reference. In order for the mic to see clean phantom power it must be referenced at the pin 1 of it's XLR, that is done with R3 and C1, R3 for decoupling from the source and C1 for cleaning it referenced to pin1 from XLR. This could be done in the board, XLR pin1 coming only for the cap, then the 3 resistors connected as they are. The other way (which I use most for my designs) I connect the phantom powering directly to the XLR connector, using the same pin from the cap (or other short wire) to connect to the chasis at the screw holding the connector.
As I decribed, the cap switch shouldn't have problems with phantom, caps are always charged as they suppose to be, 100k may be too high as bleeding resistors, in a 2k mic preamp, using 10k or 20k at that point should be good. But they are only connected to the output when they are needed. Transformer is out completely which is important. When switching the caps already have the positive side connected, so no strange things happening there, and the other side connected to ground through the bleeding resistors, so they are charged just as they need to show 0V at the input of the gain stage. What I describe is to avoid switching noise when phantom is on, when phantom is off it shouldn't be much of that anyway. It's always preferable to have caps connected to somewhere all the time, not a big deal with leaky electrolytics but a cap may maintain a charge quite some time, and if a charge is builded there for some reason and you then connect the cap you may damage something. The resistors are there to avoid that.
About the attenuator, I would use 2x1k resistor and a 220Ω resistor. This is a bit higher than 150Ω expected but I always assume that if you are using the pad is because the signal from the mic is too high to use it at the minimum gain of the amp, likely to bee too high, then I'm more concerned for the overload of the mic driving low impedance sources than the noise floor for it. Then I go for at least the most common rated impedance of 2k than lower.
JS