Ahh yeah that's a bit confusing, the merge topic tool can be used between categories, so you should click merge first and then select the topics. The placement seems like a theme/skin issue.
Only see topics that you create yourself
When we used to use IPB, we had a handy feature where general public members could enter a forum and only see the topics they have created themselves (used to be handy with ban appeals and reporting people without judgement)
Is there a way we can do this with NodeBB?
Not at the moment without a custom plugin.