Yes, you can get the demo without actually opening an account. Usually 30 days, but I know that you can extend it with Amp at least a few times without any 'sales pressure'. It will cost you your e mail address, however.
Other brokers support the platform but you must do your homework to find out which do. You do not pay for Ninja until you actually place a trade through their interface, which means essentially, that if , for example, you were using another supported data feed like IB, you can simply use their charts to monitor market movement.