Since WebCache is not included in 10.1.3.1.0 AS (SOA Suite), is it possible to use the 10.1.2 release for a 10.1.3.1.0 AS farm?
Furthermore, most documentation I've read seem to indicate a single WebCache instance for any number of AS instances, but can the WebCache instance be a multiple instance installation? We would like to use WebCache, but are a bit afraid that this might become the single point of failure.
Yes you can use the 10.1.2 Webcache for the 10.1.3. SOA Suite. However I would not advise you to do so, Most calls will be SOAP messages that will not really benefit from the Webcache.
Yes, you can cluster the Webcache. This will remove your SPOF.