probe offset value

I have a 12mm inductive probe mounted on my Ender3. It is about 4.5mm to the left of the nozzle, 1.5mm in front of the nozzle, and about 2mm above the nozzle.
(which would be in the negative X, and Y direction, and positive Z direction relative to the nozzle).
Are the offset values entered in the firmware measured from the probe to the nozzle, or from the nozzle to the probe? (IE: are they positive or negative values in my case?)

Also, when specifying the locations on the bed where one wishes to probe, are these specified as the probe location, or the nozzle location?
As I understand it, having homed the printer, it knows where the nozzle is, and can locate the probe position by applying the probe offset values.
However, when we probe the bed to perform a tramming operation ("auto bed leveling"), we need to put the probe over specific locations.

Also, due to the offset position of the probe, we can't probe the entire bed, in my case parts of the bed that in the rear and right side of the bed
(max X and Y values) are not reachable by the probe. How is this handled in the firmware? Do we interpolate to extend the slope to the 'blind' areas, or just assume that
these areas are 'flat'?

I have my probe wired to the port that would be used by BL touch. I still have the Z end stop connected. I'd like to keep the Z end stop, and use it to initially home the printer.
The z end stop is set to keep the nozzle from crashing into the highest point on the bed, so it sets up a safe starting point for the z probe operation. Can this be done in Marlin?

There's a section in configuration.h that shows how to set the probe offsets.

X & Y offsets can just be measured with a ruler. The Z offset will have to be calculated using a process similar to the following:

1. Turn off leveling
2. Lower extruder until Z sensor activates. Note the indicated height. I like to use M48 command to obtain the indicated height.
3. Lower the head until the nozzle barely touches the bed. Use the "paper drag" method to determine this.
4. Subtract the current indicated height from the one on step ? to get the traveled distance.
5. Add the traveled distance to the Z offset and save it.
6. If you have the EEPROM enabled then you'll also need to write the new value into EEPROM. This is done by issuing a M500 command. If you've made the change in configuration.h, recompiles & downloaded then the sequencewould be M502, M500.

Filling in the mesh where the probe can't reach is done by doing manual probes of each missing node. In UBL the command would be G29 P2. There are several options that can be used with the G29 P2 command. You need to look at the G29 command for all the parameters available