Can we shift from On-Demand to Reserved Instances without downtime?
It is possible to shift from on-demand to reserved instance. However, there is no option to directly change an on-demand instance to reserved. If you have multiple instances operating same website, let's say, one is reserved, other is on-demand, you can reroute the traffic from on-demand to reserved instance.
is there a way of getting alerts other than Email?
we can use amazon cloud watch for receiving notification if certain thresholds are breached. OR
Amazon SNS to send push notification messages directly to apps on mobile devices.
they are also many other services which can be used based on the situations.
What is Elastic Beanstalk?
EBS offers scalable, durable, and high-performance storage solutions for persistent data for EC2.(for example we can scale up or scale down based on the traffic and also data remains even if the EC2 instances is stopped or terminated )
What happens if we install perpetual edition of MS Office ( let us say Microsoft Excel 2021) in Sever. can everyone who has access to server access it?
yes, before that we have to configure through License Manager. we have to define the Roles and Responsibilities IAM. Based on the file's permissions user roles on the server.
can we install Win 11 etc in Server? is this feasible technically?
Yes, its technically feasible but we have to use the service "Bring your own license" If and only the license agreement meet the criteria. If not AWS doesn't allowed to install or use other services from outside.