Ólafur,

This is interesting.

First, I will say that diagrams are great when discussing rules.  Your diagram helps so much with your question.  What software did you use?

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As for your question, I think it is easiest to forget about the right gate mark for the moment.  Perhaps, the inclusion of the right mark rules is confusing things a bit.

So, at the left mark zone, Blue is inside/overlapped, and entitled to mark-room for the left mark.  By the definition, the mark-room is to sail to the mark from her position when rule 18 turned on (position 1).

Looking at the diagram, Yellow is preventing Blue from sailing 'TO' the mark throughout.  Yellow should have been giving mark-room from the moment she entered the zone (3 boat lengths from the mark).  Not 1.5 boat lengths only when Blue claims 'mark-room.

Yellow, should have borne away, sailed by-the-lee or even gybed to give Blue that room much earlier then.

Blue could have pushed her case harder, by bearing off a little.  In doing so, she would have broken RRS 11, but would be exonerated by RRS 43.1(b).  Even if there was contact, she  would have broken RRS 14, but if there was no damage or injury, would be exonerated by RRS 43.1(c).

So in answer to your question then since Blue claimed mark-room then, "Yes!  Yellow broke RRS 18.2 at the left mark.  She was required to give more room."

I hope that helps.  Let's see what anyone else says.