How to aggregate records in S3 using IOT Rules and Action?

I'd like to aggregate prediction MQTT messages generated by the Greengrass ML Feedback Connector over a certain period into a JSON-Lines file in S3: Which may then be used as a SageMaker Ground Truth input manifest, or for other analysis.

I'm not clear from the doc on the IoT Rules S3 action whether:

• It would be safe to have multiple (perhaps close/concurrent) messages pushing to the same S3 bucket & key via Rules
• Messages would be concatenated into one file (with a separator?), or overwrite
• There's enough flexibility in the IoT SQL functions to control the level of aggregation nicely (Seems like day/week/month would be tough, but probably could just do something with integer-dividing the timestamp() number

It seems like this is a use case that IoT Analytics has tools to handle, but I'm just trying to concatenate the messages together, not perform analyses/proper aggregations.