I know far less about gluing keytops than any of the respondents who have replied thus far. However, and for what it's worth, Howard Piano Industries sells a PVC glue (yes, PVC not PVA) that they recommend for keytops.  It's reportedly water soluble, clear and reversible. I purchased some for rebushing some keys on my Burton harpsichord. It was easy to use but note, again, that I have never used it for keytops.  Here's the link to the PVC glue"

http://howardpianoindustries.com/pvc-e-glue/

Bill

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