Can I use the same key pair file across all regions? I want to keep the ssh access as simple as possible.
I have set up one file for each region. I tried to upload the same file in different region but it did not not work as expected.
You can definitely use a single key pair across multiple AWS regions as mentioned in this Amazon's post.
What you do is, you upload your own SSH public key to EC2 and it is passed to new instances when they are launched.
Visit the following link to learn how you can import your own public key.