Clearance through pcb
PCBs are treated by the IEC 60950-1 &quot;separately&quot; as for solid insulation, that is when the path of discharge passes through a solid object (i.e. the substrate / prepreg).

The main requirement is that the thickness must be &gt;= 0.4 mm: that barely fits the 4-layer pcb with standard thickness of 1.6 mm. A simpler 2-layer PCB is evidently ok.
Table 2R of the standard indicates Type Tests, that have the purpose of assuring some quality in the fabrication: PCB manufacturers comply with this, although it may be difficult to get the right documentation. This type of tests was required for insulating layers other than the standard material including pre-preg, when cured below 500 deg.
Then the electric strength test is applied almost to all cases, and it is a routine test (on all samples). This test can be carried out with AC sine waveform or at DC with value equal to the peak AC.
Although the test is carried out by applying the voltage for 1 minute, when it is used for routine test (as for PCBs) the standard allowed to reduce the duration to 5 seconds, and the amplitude by 10% (when used amplitude is that of Table 5C).
I fact, there are two criteria: Table 5B based on peak working voltages (so the maximum repetitive value including DC ad AC components of the operating waveform); Table 5C based on required withstand voltage, thus distinguishing between basic insulation and reinforced insulation (factor 1.5x approx).

Going to the question on 240 Vc: yes, you can; I used 400 Vac on an old 2-layer pcb (old = 35 years ago, so now quality of materials has improved).
Going to the last point: thickness must be &gt;= 0.4 mm, no other requirements; if a material is better, well, you have a better margin but you must pass the electric strength test &amp; you must have 0.4 mm. If your material is able to work well with only 0.1 mm, nope: if it gives insulation for safety it must comply to 0.4 mm; if it were just for functional insulation, you can do what you want.