Clarification -
Please disregard my first question - the Lead Create Date is a custom column that I added in an attempt to expose the Lead was Created date (i.e. date the lead became known), and my smart campaign is not updating it in some cases.
I still would like to know why you can filter on Lead was Created (which means it must exist), but it is not displayed in the lead detail, as a column in smart lists or in the Field Management.
For my final question, I believe that when using the Lead was Created filter with leads that have been merged and have New Lead activities on different dates, you can exclude leads that also have a New Lead activity in a different period by adding a Not Lead was Created filter(s) and specifying the periods you want to exclude.