Cheat Methods:
In the Ultima 9 directory there is a file called default.kmp. When opened in a standard wordpad or other text editor you will find the following text line:

[Cheat Command]
Simply add the following lines underneath it:
alt+shift+i = toggle_avatar_invulnerable
alt+shift+l = toggle_avatar_fly

During game play the corresponding keys (alt+shift+i & alt+shift+l) will activate the cheat functions.

You may also bind the following codes just as you did the ones above:

pass_one_hour
unpass_one_hour
sunrise_sunset
pass_one_minute
unpass_one_minute
toggle_sun
toggle_wind
toggle_storms
toggle_avatar_fast

Version 1.07+
You can also add the following to the default.kmp file when you update to version 1.07 or higher:

recharge_mana
fill_spellbook
spells_0-2
spells_3-5
spells_6-8

Monsters Ignore You:
Just open up the backpack (pressing B). Monsters seem to ignore you when you have the backpack open. You can hack and slash at them as you wish then. In order not to let the backpack obstruct your view, you could drag it to the edge of the screen.