A and B: The pin is not an obstruction, it is a starting mark and mark of the course.
Neither is it a continuing obstruction.
As I live in France I willl give you the standard answer to everything : It depends. Is the water tidal? Does the white mark move significantly? How much water is there above the sunken marine hazard? How wide are these 35 feet long boats? Could any boat of that type, with a crew with an expected level of competence, manoeuvre in a seamanlike manner around the mark in any expected weather conditions/sea state?
C: RRS 86.1 A rule of Part 2 cannot be changed. To get around the whole issue, my preferred wording in the SIs would be along the lines of: No boat may cross a line between the starting mark (red channel marker) and the white float, in either direction.

Welcome Charlotte!  Happy to have your contribution. 

Ang

As a retired IJ and IU my take is that Blue, being a fast reaching multi-hull, is on the cusp of breaking rule 17 at position 6.

I think that wording is quite workable, if that's what you want to do.

Rule 89.2 provides the option to change the NoR provided adequate notice is given.

As brought out by John d Farris, any change needs to be communicated to all stakeholders.