1.) Transacting on the blockchain costs fees. It would be very expensive to fake bitcoin transaction volume. Even if you're a miner, your fake volume would be displacing real volume that would be paying real fees.
2.) Transactions on exchanges never hit the blockchain, so exchange volume and blockchain volume are unrelated. Furthermore, keeping the price high would not be "quite straightforward". What method do you have in mind? The only way to artificially keep the price high long-term is to place infinitely-large buy orders at the minimum price you want. This will obviously cost you unbounded amounts of money and would be financial suicide.
2.) Transactions on exchanges never hit the blockchain, so exchange volume and blockchain volume are unrelated. Furthermore, keeping the price high would not be "quite straightforward". What method do you have in mind? The only way to artificially keep the price high long-term is to place infinitely-large buy orders at the minimum price you want. This will obviously cost you unbounded amounts of money and would be financial suicide.