The easiest way for both the first in-text and the subsequent in-texts, are to hide the author and insert the text you want, as you want it to appear in the Prefix box (complete with a trailing space). Otherwise there is no way to insert new text between the author and the year.
For Author yeara and yearb, you edit the style (Edit>edit output style>edit “your current stylenaname”) to disambiguate the year. Those settings are in the citation settings, Ambiguous citations (see attached). Unfortunately, endnote won’t combine them unless the whole author list is identical for two records, and you may need to hide the author for the second instance to get them to appear as in your example.
Remember to save as to a new name, and then you need to make sure that newly named style is used in the Word document on the Endnote ribbon. Do not use the same name, and the first time you look for it, you need to show all styles at the top of the dropdown list.
