Hi Eric -- Congratulations on finding your Credenza! You're going to love it!
Regarding the weakness of the dashpots, Matt's advice is good: drop something like a quarter-teaspoon of neatsfoot oil down each of the tubes through the slots at the tops, where the flat bars slide in. There are leather discs inside the tubes whose mission is to control the amount of air in the tubes. Very often these have have dried out over the years, the leather is no longer supple, and the discs can't do their job. Often relubricating them with neatsfoot oil will bring them back to life. You can get this a a shoe repair store if you can't find it anywhere else. Though other oils may do the trick, neatsfoot oil is the traditional go-to for maintaining leather, so use it if you can. It's sold as both pure neatsfoot and a neatsfoot "compound", which as I understand it is a blend of neatsfoot and mineral oil. Pure neatsfoot might be better, but I don't think the compound version is much worse except that you wouldn't be able to know exactly what the blend is. Use only a little at a time, and let it sit overnight. I made the mistake of adding more oil -- too much! -- when the first dose didn't seem to "take" quickly enough for my impatience, and that resulted in a small amount of leakage from the bottoms of the dashpots: no big deal, but avoidable.
Having said that, in my case relubrication wasn't the solution to the problem, though it seems to be effective very often. Luckily, I found that there's a really good video on Youtube by a knowledgeable restorer who calls himself chilldude2007 that demonstrates in detail how to refurbish the dashpots. In it, he shows a trick of reversing the leather discs (which are actually sort of cup-shaped) so that they supply more resistance to the air inside the tubes. I did this, and my dashpots have worked very well since then. The business of taking the dashpots apart and re-assembling them seems complicated at first, but it really isn't very difficult. The most important thing is not to lose track of the few small ball bearings and springs within the assembly, so if you do have to resort to a rebuild, work (as always with small parts) over some sort of a tray.
Here's the video:
http://www.youtube.com/watch?v=QKHdzIL0mLU
As far as the holes in the horn go, assuming they're just a small number of ordinary-sized screw holes, I would probably use either a good-quality clear acrylic caulking material or perhaps something like JB Weld Kwik-wood. I think the acrylic would be more likely to accommodate ordinary expansion and contraction of the wood due to temperature and humidity fluctuations, but really, if the holes are small it shouldn't make much difference. While you're working on the horn, it might be a good idea to remove the back panels of the machine, and face the back toward a strong light. Examine the horn from the front side to be sure that there are no gaps between the horn's wood strips; the light will reveal if there are. I took the extra step of pressing a very small amount of clear acrylic into the small spaces between the joins, wiping along the joins with a damp cloth afterward to remove any residue. While you're at it, it's also a very good idea to give the horn a treatment with a 50/50 mixture of glycerin and denatured alcohol to restore some moisture to the wood. Doing this made a marked improvement in tone when compared to pre-treatment playing of the same records, enhancing the bass that the Credenza is known for. I'm pretty sure there are details of how to do it elsewhere on the forum.
My Credenza was a beat-up wreck when I got it, having been painted blue, then green, then stripped and very badly refinished, so I had to strip it again and re-refinish it. Because it was such a total rehab, mine will never be a true "collector's" machine again, but it plays beautifully (especially after Peter Wall did whatever he did to the pot-metal reproducer, which couldn't be rebuilt) and I wouldn't trade it for any other. It's great that you're restoring a Credenza that, if you hadn't rescued it, might well have come to no good end. I hope you'll enjoy it for many years to come!
. If you wouldn't put it on your violin or guitar, don't put it on the horn! (Although granted, string instruments usually have multiple coats of sealer and varnish---but not glycerine, AFAIK.)