A fairly common starting tactic for some skippers in some fleets is to reach below the line on port and "tack under the marshmallow". The proper defense in that situation is "don't be the marshmallow". If you leave a hole between you and the boat to leeward big enough for another boat to squeeze in, then one probably will.
If I understand your (purely hypothetical) scenario correctly, the rules that apply are (in order) 12, 11 and 15, and then 11 and 16.1. Rule 14, of course, always applies.