More to play devil’s advocate than anything, but isn’t a card like arachnus web similar to Fog? Where instead of taking all creatures out of combat for a turn, you’re taking one creature out of combat for all turns?

If it just prevented combat damage, maybe, but it does more than that.

Would Arachnus Web be a bend if it required the caster to control a spider?

It being favorable, doesn’t make it any less of a bend. Almost any mechanic can be flavored.

Would Arachnus Web have been OK if it required a creature as part of the cost? Eg requiring you to tap a Spider you control in order to cast it?

It’s not just an issue of requiring a creature. Green doesn’t take away abilities from other creatures.

You said the difference between a bend and a break is a bend doesn't undercut the color's weakness, while a break does. So how is Arachnus Web a break?

One of green’s biggest weakness is being unable to kill creatures without using its own creatures to do so. A creatureless green deck should be unable to answer troublesome creatures. Arachnid Web does exactly that and thus is a pretty major break.

What about Arachnus Web and Beast Within? Are they out of green's colour pie too?

Yep.

I love Krasis Incubation with a fierce passion, but I'm curious if there's any concern on your part about it being a color-pie bleed? Blue and green have had effects like this (Arachnus Web, Ice Cage) but always with a drawback.

It’s completely within green/blue color pie. Note that just because we haven’t done something exactly before doesn’t make it okay within the color pie.