the first error in the log just indicates that there were two concurrent write operations to elasticsearch on the same process instance document, but that is no issue as writes are retried on these conflicts.
The second error is related to the websocket that is used to push status updates to web clients and not to the import.
To have have a closer look on what happens during the import of the dataset you could increase the log level with adding:
<logger name="org.camunda.optimize.service.engine.importing" level="debug" />
./environment/environment-logback.xml log configuration.
This should give you some log entries like:
17:52:33.893 [Thread-12] DEBUG o.c.o.s.e.i.f.i.VariableUpdateInstanceFetcher - Fetched  running historic variable instances which started after set timestamp with page size  within  ms
17:52:33.904 [Thread-12] INFO o.c.o.s.e.i.s.VariableUpdateInstanceImportService - Refuse to add variable [approverGroups] from variable import adapter plugin. Variable has no type or type is not supported.
17:52:33.904 [Thread-12] INFO o.c.o.s.e.i.s.VariableUpdateInstanceImportService - Refuse to add variable [invoiceDocument] from variable import adapter plugin. Variable has no type or type is not supported.
17:52:33.910 [ElasticsearchImportJobExecutor-pool-0] DEBUG o.c.o.s.e.w.v.VariableUpdateWriter - Writing  variables to elasticsearch
if Optimize is able to query variables from the engines API.
Would it be possible for you to do an import from scratch, meaning deleting the elasticsearch optimize indexes and restarting Optimize?