its got a Fanuc control, but maybe someone here can help?

My Boss recently purchased a 2008 Okuma VTM-120Yb machining center with a Fanuc 18i-MB control. im self taught and have been working in the cnc game for the better part of 20 years but this one is giving me a headache.
ive got the machine up and moving around a bit, made a few cuts via MDI and all seems ok with the machine. but ive got some questions.

1) establishing work locations and tool lengths? i made a "probe" to use for touching off the work, for simplicity sake i left the tool length offset for the probe as zero. i touch the top of the work with said probe and establish it as zero in G54. x and y are zero for now.
the problem is, when i go to touch off my next tool. i bring the tool into contact with the work, and set the length based on the absolute display, and then send the tool "home". the issue is, when i send the tool back to ZO, it goes right past zero to about -1/2"

2) home positions (G27, G28, G30) what are these actually used for? when i try to use G27 the machine just goes to X0 Y0 and alarms out. G28 appears to be tool change position. and G30 is "home". these books say i dont need to send this machine home before operation so not too sure about the use of these

its also worth mentioning that when i touch off any work or tools, the values im having to input or "measure" are in the -20ish inches, which doesnt seem correct. any help would be very much appreciated as the machine came with almost zero for literature.

3) the machine has a tool setting arm, but i havent been able to find ANYTHING in relation to how to use it. so a shove in the right direction would be helpful with regards to that as well.