Frankly this thread has too many questions in it for a single poster to be able to answer it all fully in a reasonable amount of time...the stuff about lending bitcoin at interest, trust issues, etc. should probably be split off into a separate thread in the Services section.
As far as possible hoarding of bitcoin goes, if a single entity were able to buy up a large amount of BTC this would simply benefit the rest of us BTC holders because it would significantly limit the supply of BTC in circulation, thereby theoretically leading to a further increase in the value of each BTC. Taken to the extreme it would be like the Uncle Scrooge comic book story I read back in 1979 where the Scrooge McDuck cartoon character tried to buy up all of the 1916 quarters in circulation at the time by offering everyone 50 cents per quarter. He was apparently successful at limiting the supply this way and driving the price all the way up to "ten skyrillion dollars" (I am not sure exactly how much a "skyrillion" is but I imagine it is roughly equivalent to the national debt of the USA). The problem with our Uncle Scrooge was that no one could actually afford to pay 10 skyrillion dollars, so in the end he was stuck with a big pile of quarters that he could not sell because his penchant for hoarding had driven the prices too far out of whack.
In the case of our bitcoin economy it is unlikely that things would ever reach this point because there are enough hardcore "hodlers" out there who will not let go of their BTC even as the price rises, such that no one would ever be able to buy up all of the BTC in circulation. If the available supply did become sufficiently limited through hoarding and/or increased demand, then smaller units like satoshi could become valuable enough to necessitate a few more decimal places being added to the displayed balances of bitcoin wallets and transactions. I am not sure how easy or difficult it would be to add more decimal places than the current eight to the underlying code, however; for the answer to this, and the questions about BTC lending issues, we will see if other posters have something more to add.