i would just drill another hole a couple of inches from the anchor wire[just big enough for the vhf cable] i would buy the bigger vhf cable and seal it with a good sealent[apply some to the cable before pulling the last couple of inches through,if your new hole is a slightly tight fit i doubt you will have a leakage problem,a stern mounted antenna on a sailboat doesn't work well at all
Depends on the situation. I've had a stern mounted antenna for years. I just found it easier to deal with than a masthead mount and the extra connections and sealing necessary with annual unstepping/stepping the mast. Mounting it on the stern rail makes for simple connect/disconnect with the seasons and simplifies troubleshooting if something goes pear-shaped.
Range is diminished, but depending upon where and how you sail it might not be a big concern.