You will have to take the deck off.
I use a yard crane to lift the mower off the deck ( I am 68 ).
Put some 8' long 2x4 under the deck then lower the deck on them to undo the deck then slide the deck out on the 2x4's.
Soooo much easier to do.
Yes they now make zerks with a spline on them that are literally hammered into the holes.
Cub use them , don't know who else .
I just drill the spindle when doing the bearings and replace the zerk with a normal threaded one.
The nozzle on the end of the grease gun should lock onto the zerk if not get a self locking one