OK, here we go.
1. Those unzip errors are not related to this. You propably have some extra files what it's trying to deploy.
2. This loads to my 4.x instance without errors. So, I would suggest you to try this on clean HQ installation.
3. I believe there's something what needs to be added to xml file:
I would use <config include="snmp" />. I don't exactly know what that include, but you may have already manually added those configs.
I think server section also needs this
<plugin type="measurement" class="net.hyperic.hq.product.SNMPMeasurementPlugin"/>
definition. If you later want get metrics from there, I think.
Last what comes to my mind, is filters. I think snmp measurement wont work until some filters are introduced(you see these within every plugin). I've seen those used in snmp plugins. At least you are setting template on your metrics, so this may not be that big issue.
I had a plan to do plugin for our printers and ip-phones. I don't need it, but my colleague wants to have it.
But, first you need to be able to load your plugin without errors.