"Harmless" is a keyword ability which affects the process of dealing damage with this object as the source. Rules 120.3a-e do not apply (i.e. it does not affect the permanent or player that the damage is dealt to; any additional rules made up custom or future that do so are also suppressed) (rule 120.3f still applies if it has lifelink), and rule 704.5h (the state-based action for deathtouch) does not apply to this damage. This does not prevent the damage from being dealt, even if it has no effect.

Lethal damage, excess damage, and combat damage assignment, ignore the harmless keyword; they are computed in the same way whether or not it has this ability.

Huh. Interesting - so basically doesn't get counted against toughness or life?

And then; then you add in banding. Which is just maximal confusion on it.

I'm not sure this is needing a keyword, when we already have the wording "~ deals damage in the form of" and "If ~ would deal damage, (you may) instead"...

Bu on the flip side; this could replace most of those phrases. I'm torn.

Reading the rule on excess damage, I notice it doesn't seem to properly/intuitively address two creatures with deathtouch and trample being blocked by the same creature since it only checks the damage source for damage not the source of any other damage dealt to the same creature.

I think it should be "a source dealing damage" not "the source dealing that damage".

Makes you wonder how deathtouch + harmless really interacts with trample.

Harmless doesn't change the amount of damage that would be dealt. It only changes how damage is represented, or not represented actually. 

Thus it wouldn't affect combat damage assignment, as explicitly claimed by OP. including that trample has no change or affect or interaction with harmless.

Deathtouch has two phases. First is damage assignment. Again this phase has no interaction with harmless. Second phase is state-based effect. This is when harmless is checked.