You'll only be able to use tstats if the fields are indexed. By default, that is host, source, sourcetype and _time. If eventName and success are search time fields then you will not be able to use tstats.
You can quickly check by running the following search
| tstats count WHERE sourcetype = expwebtracelog (eventName=* OR success=*) by eventName,success
If you get no results then these are not indexed fields.
If your data model is accelerated you might be able to use tstats for your first query, try
| tstats prestats=t count(Service_Events) WHERE eventName=service:SoftvoyageService* AND success=false FROM Expweb_Tracelog_Service Service_Events by _time | timechart AS count(Service_Events) "Count of Service_Events" | head 100
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