Picture is missing, but I’m sure it’s lovely! 
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Yes, you can add text in Inkscape - convert the text to outlines/curves before you engrave it. Inkscape Text to Outlines
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I still use wood glue (I prefer it for bonding) but rough up the surfaces a touch with an 80 grit sandpaper first. (Takes real guts to do that the first time…marring that beautiful finish deliberately is a heartbreaker.) You can also use CA.
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Yes, if you do, you need to align to the Preview, not the cutout, and make sure you haven’t yet moved the cut in any way.
How to Add something you Forgot into an Open File. 🤔