Like Larry mentionned previously, it depends on your level. It's normal to have your spoken fluency decreased as soon as we stop practising a language regularly. That was my case with French when I was in China submerged into teaching Russian&learning Chinese, but as soon as I had to revive French, I could do it quite quickly, within a couple of months. The thing is that if you want to progress fast in your new language, you have to drown yourself in it forgetting about other languages. At least it'd always worked like this for me. But recently I've discovered that it's actually possible to maintain other languages doing some background passive activities like reading/listening/watching without focusing much on learning and tackle a new language more actively. It'a bit more difficult to keep the pace and motivation though. But maybe it's just me, because my motivation is fluctuating dramatically...

I think - no! If you know Spanish good and will repeat it from time to time then everything will be ok! :)

If you are going to disregard your Spanish for a year, be prepared that it is going to deteriorate. But of course you can learn a new language and still do some occasional activities to maintain the languages you already know.

Not at all. I have a friend, who live in Russia for 3 years, and still remember he's own language. If you would live here for 10+ years you can forget something, but even in this case it will not decrease a lot) A year will do nothing to your language)

It certainly depends on your current level of Spanish. I would suggest reading at least a book a month in Spanish to maintain proficiency. I lost most of my Spanish, but it was over a 40 year period of little use. And, I did not have a good foundation. Had I had a good Spanish foundation, I would not have lost so much.