It's not ProximityHash that does the optimization. There's an option to use Georaptor which does the optimization. Please have a look at GeoRaptor (https://github.com/ashwin711/georaptor).

On your second question, it completely depends on the problem at hand. I have been using GEORADIUS on Redis for around a year now @ > 30k QPS. The case there is to find locations that are closest to our input. ProximityHash provides you all the geohashes in a circular area for which I haven't found a library yet. Most of them provide geohashes in a BoundingBox.