* Source templates and websites

[tatewise]

Ok, for some of your questions I can give answers but others are more of a challenge.

"how do I know that {%CUR~CITN.PAGE%} and {%CUR~CITN.DATA.DATE%} are valid?"
That is one of the challenging ones.
In Tools > Source Template Definitions... when editing a template the << Add buttons, that could offer all the valid data references, only offer metafield references for the Field Codes defined below and not the generic fields.
To discover the generic references you need to use another feature dialogue and its Data Ref Assistant.
e.g. Use a Query and on the Columns tab interrogate the Fields on the left for Source sub-fields such as Where Within Source (%INDI.SOUR[1].PAGE%) or Data + Entry Date (%INDI.SOUR[1].DATA.DATE%).
Maybe that needs reporting to CP that generic data refs are not offered for template Formats via << Add button.
It is perhaps that FH doesn't want you to use generic fields.

The Source Template Definition dialogue has tools for managing the Collections of Templates held outside Projects.
e.g. Import/Export, New/Edit/Clone/Delete, 'Compare/Sync with Source Template Record' essential in handling updates, and 'Add to Project' which puts the Source Template records into a Project and shown in its Records Window.
The Source Template records have none of those tools and managing changes to them would be very difficult.

{%CUR~CITN.PAGE%} refers to the generic citation GEDCOM field Where Within Source, shown in your earlier screenshot SNAG-0008.jpg and will be understood in other products that handle GEDCOM fields.
{Page_Etc} refers to a defined metafield Page Etc which in the SNAG-0009.jpg example below is Citation-specific and is prefixed with a bullet character. Such fields are an FH invention and only understood only within FH.

You misunderstand what Add Source Citation does.
The option is actually Add New Source from Template which creates a new Source record (on Sources tab) linked to a Source Template record that provides the definition of the metafields and footnotes, etc.
The Source record holds the actual data values for each field and is linked to Facts via Citations.
When you use the Edit Source Citation button to show the yellow Citation Window it is those Source records and Citations that are edited. The format of the metafields and footnote and bibliography are defined by the Source Template record linked to the Source record. The same Source Template record can be linked to many Source records that all utilise the same definition.
So using the Add Source Citation button 5 times creates 5 Source records not 5 Source Template records.