Best programmatic solution to push changesets from local PostGIS OSM replicant

JOSM has the option of reading shape files and converting them to OSM data, and there's also a number of shp2osm-style programs around, but none of these techniques actually allows updating existing objects. Doing so will require, among other things, conflict resolution for cases where the object has been edited in the mean time. You might want to check out the codebase of "POSM" here https://github.com/AmericanRedCross/posm which has the aim of allowing offline edits and loading them into OSM later; perhaps there's something you can re-use.

Having said that: You write that you have "generated" a nearly complete set of data. If this involves (a) the automatic conversion/use of third-party data sources and/or (b) the query-based or rule-based editing of OSM objects (e.g. search+replace operations on names, geometric operations like merging tags from nodes to ways or vice versa, removal of duplicates, etc.etc.), then you must follow the appropriate policies for imports and/or mechanical edits. The most important point of both these policies is that you must discuss your import/edit with the community before you act, and you must explain what rules you've applied to what data. If you simply stuff the results of some computation into OSM then your edits will be reverted. The same is true if you plan to continually apply automatic edits - seek community agreement before you act. The reason we have these policies is that a lot can go wrong, even if people think they know what they're doing.