Birch, including Silver Birch, are notoriously poor woods to store in any way. Quick growing, porous qualities, and soft in nature they are, as you've noticed, very quick to spalt and degrade. Sealing will not prolong their usefulness in my experience.
If I have birch I always work the principle that it should be worked as near from green where ever possible, and if spalting is required then three months after felling is the latest I'll use it. After that it's pretty much poor grade firewood.
Don't forget that if you're using any wood whilst the spalting is effectively active then there is a potential risk of pulmonary infection from the spores, so microwaving to kill the spores is a sensible precaution. You could also use a proprietry anti-fungal treatment, but this can cause finishing problems later, and ensuring deep enough penetration of the wood can be an issue. There have always been issues surrounding spalted woods, and more often than not they've been ignored, mainly because we all used kiln dried spalted blanks for a long time, and in these the active spores are dead, but wood with live spores/fungus can be an issue...so if using them take advice or search for information.