To live stream video content to store in S3 (not to stream the content off S3), can Elemental MediaConvert or Interactive Video Service be used?

Amazon IVS can be used to record a live stream directly to Amazon S3. IVS pricing is shown here. You can then serve via a CDN like CloudFront, which has costs associated.

https://docs.aws.amazon.com/ivs/latest/LowLatencyUserGuide/record-to-s3.html

You can also use Elemental MediaLive to record to S3. Here is a guide that includes a cost estimate:

https://docs.aws.amazon.com/solutions/latest/live-streaming-on-aws-with-amazon-s3/solution-overview.html

Capturing a live stream into S3 would require AWS Elemental MediaLive, Amazon IVS or solution (DIY or 3rd party) that does this.

AWS Elemental MediaConvert is a file transcoder and will not work with live sources. It can transcode a captured stream to other video formats/wrappers if needed.

AWS Elemental MediaLive can write either a Transport Stream (.ts) to S3, as an archive file, or HLS, with manifests and segments. Either of these outputs could be transcoded by MediaConvert to, perhaps an MP4 file if desired. Using AWS Lambda functions, a MediaConvert transcode job could be initiated when MediaLive writes to an S3 bucket.

Amazon IVS can also capture a live stream to S3 as HLS. Currently, this would only be for RTMP sources.