I don't want to say I'm an expert by any means but I've mill maxed about 9-10 PCB's and 9 of them were new PCB's minus the one which was my personal Space65 R1 PCB that had soldered switches

1. If de soldering switches for sure make sure holes are clean of any residual solder, sometimes if some stays around the PCB switch holes and you cant get the mill max socket in you can try placing the heated solder tip on top of mill max socket and let it heat the residual solder in the pcb. You should feel the socket fall into place.
2. When placing sockets into PCB you can lock them in place with masking tape, I normally place all sockets in one row at a time and then cut some masking tape to the correct width to place on said row to lock them in place. I'd then move down to the next row and do the same, *caution* make sure you have the correct placement for your sockets when doing the last row / steps cap / enter / backspace etc
3. When soldering mill max sockets this has always worked for me in the past. Place the solder tip at the base of the locked socket first for 1-2 seconds before placing the solder up to the base of the socket, I've found this to work for me 100% of the time when soldering 0305 and 7305 sockets.