Clearance of Exposed Nodes on High Voltage Relay
You should distinguish between safety specifications and operating specifications.

25 mm clearance or creepage for 3.5 kV is a safety specification. For comparison, IEC-61010 (a safety standard I happen to have handy) requires 10 mm clearance and 12 mm creepage for 3.2 kV AC (under certain specific material and "pollution" conditions). You can see the IPC spec is much more conservative than the IEC one. IPC is also not a legal requirement in any jurisdiction I know of.

The dielectric strength of air is about 3 kV/mm, so you are not going to get arcs between pins until you bring them closer than ~2 mm from each other.

You might see leakage current through contaminants on the PCB surface if you cannot clean your PCB and keep it clean in operation. How much this affects your circuit depends on the exact nature of your circuit and how much contamination gets on your board.

If you're going to sell your design, it would be a good idea to find out exactly what regulations you are subject to and get hold of the relevant documents. Or hire a PE with access to the appropriate documents to advise you on the required creepages and clearances for your specific application and market.