Yes. MongoDB keeps most recently used data in RAM. If you have created indexes for your queries and your working data set fits in RAM, MongoDB serves all queries from memory.

Because a single document can contain related data that would otherwise be modelled across separate parent-child tables in a relational schema, MongoDB’s atomic single-document operations already provide transaction semantics that meet the data integrity needs of the majority of applications. One or more fields may be written in a single operation, including updates to multiple

Not directly, no. However, MongoDB does support a rich query language of its own.

For the list of supported platforms, see Supported Platforms

As a best practice and the recommended way cluster should have minimum three nodes. Primary with Two Secondary Members (P-S-S) These deployments provide two complete copies of the data set at all times in addition to the primary. These replica sets provide additional fault tolerance and high availability. If the primary is unavailable, the replica

OpsManager is part of Enterprise subscription. as per the Subscription Policy Mixed Environment is not supported.

Yes., there are few Prerequisites and configuration needed to setup OpsManager. In this Webinar we have covered this in detail. You may like to watch this on-demand. please refer: https://ashnik.com/resources/webinars/how-to-automate-deployment-monitoring-and-manage-mongodb-clusters/

On AWS, you have an option to configure mongodb from Micro., you may like to choose your requirement of the workload. For Complete AWS EC2 Instance Types. Please Refer: https://aws.amazon.com/ec2/instance-types/