Explanation
Box 1 = On (no default)For con-iot1, you need to configure the default TTL setting to On (no default), which means that items in this container do not expire by default, but you can override the TTL value on a per-item basis. This meets the requirement of retaining all telemetry data unless overridden1.
Box 2 = On (3600 seconds)For con-iot2, you need to configure the default TTL setting to On (3600 seconds), which means that items in this container will expire 3600 seconds (one hour) after their last modified time. This meets the requirement of deleting all telemetry data older than one hour1.

Explanation
Bounded staleness is frequently chosen by globally distributed applications that expect low write latencies but require total global order guarantee. Bounded staleness is great for applications featuring group collaboration and sharing, stock ticker, publish-subscribe/queueing etc.
Scenario: Application updates in con-product frequently cause HTTP status code 429 "Too many requests".
You discover that the 429 status code relates to excessive request unit (RU) consumption during the updates.
Reference:
https://docs.microsoft.com/en-us/azure/cosmos-db/consistency-levels

Explanation
Azure Cosmos DB supports queries with parameters expressed by the familiar @ notation. Parameterized SQL provides robust handling and escaping of user input, and prevents accidental exposure of data through SQL injection.
For example, you can write a query that takes lastName and address.state as parameters, and execute it for various values of lastName and address.state based on user input.
SELECT *
FROM Families f
WHERE f.lastName = @lastName AND f.address.state = @addressState
Reference: https://docs.microsoft.com/en-us/azure/cosmos-db/sql/sql-query-parameterized-queries

Explanation

Box 1: No
Azure Cosmos DB's change feed is a great choice as a central data store in event sourcing architectures where all data ingestion is modeled as writes (no updates or deletes).
Note: The change feed does not capture deletes. If you delete an item from your container, it is also removed from the change feed. The most common method of handling this is adding a soft marker on the items that are being deleted. You can add a property called "deleted" and set it to "true" at the time of deletion. This document update will show up in the change feed. You can set a TTL on this item so that it can be automatically deleted later.
Box 2: No
The _etag format is internal and you should not take dependency on it, because it can change anytime.
Box 3: Yes
Change feed support in Azure Cosmos DB works by listening to an Azure Cosmos container for any changes.
Reference:
https://docs.microsoft.com/en-us/azure/cosmos-db/sql/change-feed-design-patterns
https://docs.microsoft.com/en-us/azure/cosmos-db/change-feed

Explanation

Box 1: Yes
Need to join readings and devices.
Box 2: No
Only readings is required. All required fields are in readings.
Box 3: No
Only devices is required. All required fields are in devices.