How series resistors influence clearance-creepage requirements?
As already stated by Felthry and winny, the spacing within the resistor terminals on the high voltage side of the circuit cannot be counted as a supplementary clearance/creepage distance across the isolation barrier offered by the optotriac \$\mathrm{TRI}1\$. The reason was already guessed by you: resistors do not offer galvanic insulation between their terminals, but only a functional one, meaning that they are properly spaced in order to work with an applied voltage sufficient to make them reach their maximum power dissipation without being destroyed. As long as no current flows through them, the highest voltage on the controlled load will appear at the optotriac anodes, leaving it as the only barrier for safety/galvanic isolation. Therefore you have two choices for solving your creepage/clearance problem:


If you need a clearance distance (i.e. across the air, "at sight") larger than 2mm, you should change the optotriac model you are using. No matter how you design your control and load circuit, there will be always a condition where the maximum voltage to be "blocked" will be present across the optotriac, and it will cause an arcing discharge through air to reach your input control circuit, usually with unwanted effects, to say the least.
If instead the clearance required can be relaxed, and you instead need absolutely to have a creepage distance (i.e. along a path laying on the board/component surfaces) larger than 2mm, then you can also decide to make a cut of sufficient thickness on the board just below the body of the optotriac. The requirement will be fulfilled (it depends also on how crowded is your board), with a modest increase of board costs.