Insert Text in Credit Field

Ad 1.
I do not know how to do this using a web source script.
But you can try to modify your web source script to output the literal text ...
'Technical Credits:'$char(13)$char(10)
... just before outputting the credit info to be extracted from the web source.
See also:
[WS] Discogs (pone mod)

Another way would be - when websource work is done - to use Mp3tag for what it is made for - use an action "Format value".
See Forum FAQ how to prepend resp. append some text to an existing tag-field content.
Actions

Ad 2.
You can change the content of the tag-field ARTIST using the Mp3tag Tag-Panel for all selected files at once.
Just type the new text into the edit field and press [Ctrl]+[S] to save resp. click the 'Save' icon.
Another way is to use Mp3tag for what it is made for - use an action "Format value".

To make sure to work only on files, which have a tag-field "CREDITS", you can set the Mp3tag Filter to get only those files displayed, where the tag-field CREDITS exist.
NOT "%CREDITS%" IS ""

DD.20110530.0652.CEST