Mongodb: Can I create an unshard Replica set for a sharded cluster

I will try to be brief. Most of the answers are documented.

Q1: There are many ways. Shard a collection using _id:hashed and set initial number of chunks 2 and then start inserting documents (100 is a good number of docs). You will see documents on both shards.

Q2: It uses range partitioning. Each shard holds ranges of the shard key.

Q3: Sharding is an automated process, all you need to do is to determine a shard key.

Q4: Stop the balancer, backup shard rs1, rs2 and the config server, start the balancer.

Q5: On a sharded cluster the access point should be the mongos

Sometimes, the 3 config server are out of sync. What does it mean? How can I quickly get rid of these issues?

Most convenient way would be to copy (cp or rsync) the configdb data directory content between hosts in cases of inconsistency

When should I choose MongoDb Shard with Replica Set vs MongoDb Replica set only?

Replica ONLY: When you desire to make your mongo data highly available and add redundancy in your solution. The benefit can be to offload reporting requests to secondary nodes and/or make mongodb cluster sustain network partitions or disaster recovery

Sharding ONLY: When you desire to scale your mongodb cluster horizontally to achieve load balancing and distribute traffic to multiple hosts you must choose sharding. For example, you can achieve partitioning benefits with sharding enabled on databases.

Sharding with replica set: When you desire to enable all benefits stated above i.e. scale you cluster horizontally to handle large volumes of data for load balancing and also achieve HA within each shard you must choose this approach