The plural "-er" is a 'eh/shortened ah' and a tapped 'r' (to me it sounds like a soft d). Forvo doesn't have "bassenger" yet, but other words ending in "-enger" are "trenger" (needs) [http://forvo.com/word/trenger/#no] and "senger" (beds) [http://forvo.com/word/senger/#no]. Whenever "-et" is added for a 'definite neuter', the 't' is silent, so the word ends in an 'eh/shortened ah' sound. An example is "huset" (the house) [http://forvo.com/word/huset/#no].
I was also confused. At least the English translation sounded strange to me. It was beyond me :) Fortunately, we have dictionaries. The one I consulted said that "bortenfor" means "lenger bort enn", i.e. "farther away than". I hope this may help someone to understand it better.
Not necessarily. It would be if you were standing in front of the house, but if you were standing in the back yard, it would mean in front of the house.
More likely, you'd use this if you were giving instructions to find the public pool, for example. "Beyond" is the best translation.
"Beyond the house" does not necessarily mean inside the same property. It could mean that you have to go beyond the house (past the house) to get to a specific pool. Not a phrase we use very often. "Behind the house" would most likely mean that the pool is in the backyard, right behind the house, within the same property.