Using GarageSale Version 7.0.14b5 (834), macOS Version 10.12.6 (16G1314)
Text Snippets should work the same in Editor Mode and Preview Mode.
They should work just like Paste-ing text, replacing any selected/hilighted text. When inserting a Text Snippet (either from the popup menu or the contextual menu), delete any selected text, insert Text Snippet at the cursor insertion point, and place the cursor right after the inserted Text Snippet text. Preview Mode works much better, but both modes should work the same.
Both Editor Mode and Preview Mode should have the same Context menu “Insert Text Snippet” menu item.
Neal