Haven't tried this personally, but maybe you could also install each version of SQL Extensions into a new schema (based on version number) in the same database, & point the synonyms to those instead.
Of course this is assuming IBM ever releases a new version of SQL Extensions for Netezza (I wouldn't hold your breath waiting for it)
I recommend installing SQLEXTN in a standalone database and adding "search_path = <database>" at the bottom of the "/nz/data/postgresql.conf", which makes it local to each application database.
The readme file will not be updated in future. Will see if it can be removed in future releases.
Regarding 64-bit, no need to migrate SQL toolkit and INZA from 7.x. You just need to install the latest toolkit from fix central (no need to recompile). I agree with you that this information should probably be available upfront. Its documented here SQL ToolkitNo changes in SQL Toolkit between 7.2.1.x and 11.0.x.x. Any changes in future will be documented in the release notes on the knowledge center hereINZAINZA 11.x mostly has defect fixes. INZA release notes are available here