>1. Is it unheard of that a chapter contributes a greater amount of marines to the Deathwatch than usual? From what I understand, fluffwise, only a few battle-brothers are seconded to the Deathwatch every centrury or so (don't quote me on the exact time though, I've been unable to find that out)
There are a few chapters, like the Mantis Warriors, which make a point of sending as many Brothers as they can spare to the Deathwatch. (First Deathwatch novel has the Mantis Warriors completing their redemption in service to the Deathwatch, two brothers becoming posthumously inducted after the events in the book.) It depends on how the chapter sees membership in the Deathwatch--to some it is an onerous burden, while to some successor chapters it is the only way they can prove their honor and worthiness in the greater hierarchy of the Adeptus Astartes. Having a Brother accepted to the Deathwatch from a chapter that previously hasn't had any inducted is considered the first sign that the chapter has "made it."
>2. Can battle-brothers who have returned to their chapter still wear the shiny silver pauldron or is that pauldron exclusive to their time in the Deathwatch?
The silver pauldron is a badge of honor akin to the Crux Terminatus. They wear it always. In addition, according to the novels some leave their armor painted Deathwatch-black when they return to their home Chapters, causing some friction, just because they want to be ready to return to service the instant the Deathwatch calls them back.