Keywords and reserved words: What's a poor boy to do?
The best answer is what @akuzminksy mentioned: escape the fields.
Since you indicate doing so would be prohibitive in terms of amount of work required, there might be another solution: a pre-parse query rewrite plugin

A preparse query rewrite plugin has these characteristics:

The plugin enables rewriting of SQL statements arriving at the server before the server processes them.

The plugin receives a statement string and may return a different string.



Of course, that requires upgrading to 5.7! And the effort required to build this custom plugin might also be prohibitive.
Other tools that offer query rewriting capabilities are:

ProxySQL
MaxScale
ScaleArc

Since these tools are proxies, all rewriting is done 'preparse'.
Disclaimer: I make no claims that this will actually work, but is the only path forward I see besides code overhaul.