Chief Mountain is the crossing we used in 2009, no problems with the drive. We stayed just two nights, in Waterton itself, before heading on to Banff. June is fawning season for Waterton, the town is overrun by female deer, who come into town to give birth, as being safer from preadators.
You can get the Discovery pass as you enter your first park, with a credit card, in this case Waterton or just order it now via http://www.pc.gc.ca/eng/ar-sr/lpac-ppri/ced-ndp.aspx
Have driven into Canada three times in recent years. Currency exchange tends to be expensive. I've just gone to the ATM and withdrawn an estimated amount of cash needed. Places will also take US, at a rate favorable to what you would pay to exchange.
There are several ATMs in Waterton, at least there were in 2009.