Designing to 61010 is very difficult. I find it's a poorly written standard with everything from lasers, xrays, multimeters in there.
May be best to collect and write down several questions, and have a meeting or conference call with the agency to get answers during a technical consultation. It's very costly to go down the wrong road. Take my ramblings very lightly.
For a secondary working voltage of 24V, I would follow:
Table 7 and K.13 gives PCB creepage 0.04mm/1.6mil
Table 6 gives clearance of 0.52mm/20mil (PD2). There is a minimum clearance K.2.2 of 0.2mm (PD2).
If product size is an issue, Annex H and conformal coating (takes you to PD1) or potting can lower the 0.52mm, or you can route using PCB inner layers as they have insulation.
I'm hearing the product is still safe (under one fault) with the 24V rails floating to hazardous live. There are no user-accessible parts under normal use. This is a black box with only mains coming in, is this correct? (it seems unusual, sensor/field wiring or a LCD touch screen or buttons, switches etc. I would expect).
The plastic enclosure as supplementary insulation may be a troublesome approach as the electrical specs and approvals are always not there to qualify it as an approved insulator. I don't know how big a company you are, if the enclosure is "off the shelf" or your custom design.
I've had to prove the plastic used, thickness, flammability, rigidity, tracking index, etc. - and that all (tolerances) of it was controlled by the enclosure manufacturer, that they could not change anything without you knowing.