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Opinions on replacing bellows pins with wood screws or nails ?

Good advice above, except that I think it would be better to let the wood dry completely.

The standard method of the removal of dents in timber is to swell the dented area with water. The effect can be impressive, with the dent just vanishing. Water swells timber! That's how wooden barrels are watertight: no glue or sealant is used. There are two requirements, though: firstly, the water must be able to soak into the timber, so nothing must be around to waterproof the timber; secondly, the timber must be free to expand.

The second requirement, if not met, is what causes axe heads to become loose, if the handle in the socket gets soaking wet. The timber tries to expand, the head prevents it, so the dry timber, surrounding (deep to) the wet timber is crushed. When the timber dries, the swelling relaxes somewhat, but the crushed timber remains crushed, and the head is loose. This also means that a used, but dried-out barrel leaks like crazy! So, for this reason, it's not a good idea to insert pins into wet or damp wood. Incidentally, grubbing axes (mattocks) are made with heads that are readily removable. Why not fix them with wedges, like hammers or felling axes? Because they are used in wet and muddy conditions; it's good practice to separate the head from the handle when work is finished (and operator exhausted), to allow the wood to dry out.

Clearly, it's not a good idea to wax, grease or oil pins or holes, if the water treatment is ever going to be successful at a later date.