Lowering the interior RH slightly with a dehumidifier would make the wood less tasty, but RH drop must be slight and slow only, else wood can warp.
I would also line the feet of the pews with something impervious to stop the legs getting damp. Not often one sees furniture with a DPC I'd pick something that wont scratch or stain the floor, so maybe a thin layer of something impervious with a little wood below that. The little slips of wood would be sacrificial, but you can insecticide them first, and perhaps pick a species more resistant to woodworm.
If there are slates missing, fixing that will reduce RH, and dry off the patches under them where water gets in. May be all that needs doing.