1. I don't believe it's possible to lose your silk enchantment if you use an anvil. regardless, when you put your items in the anvil, you can move your mouse pointer over the anvil output and it will show you what enchantment you'll get.
2. Both naturally occurring and placed spawners cannot be moved, even when using a silk touch. This applies everywhere, not only in the mining world.
3. I only use silk touch on emeralds and diamonds. Emeralds tend to sell for more in their ore form (at least it has been the case IMO). If i have a fortune III pic, i'll use them on diamonds, if not, i use a silk touch to obtain the diamond ore until i have a fortune III to break them.
That's my two cents. Hope it helps!
Max