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In answer to your questions, please see the following:
1) There should be no difference. The summary calculations in the Data Set Columns expose what are called PI Summary datasets, which could be used instead of the equivalent PE functions in a PI Expression dataset.
2) Yes it does. Datasets are calculated at the specified time interval to provide a set of values over the time range of the Symbol(s) that use them, such as Trends.
3) Yes. If the calculation dataset was requiring a large amount of archive data (for example TagAvg expression over 1 year), then this helps to reduce the number of archive calls for that data by not recalculating the dataset on every DisplayRefresh interval.
4) Yes, this is an offset from midnight. It's used to set the timestamps of the calculation intervals that you see in the Preview pane in the bottom left corner of the dialog.