when I see that whichever instance I choose is defederated from another bunch of random instances, is it possible to have those show in my feed without making a separate profile from other instances? and can I turn them off again if I see that defederating was the right idea?
If the instance defederates, users do not have an option to interact with the defederated instance anymore. The only choice you have then is to join an instance where the moderation policy agrees with your values or host your own instance. If you just want to see if defederation was the right idea, you could just visit those instances directly and look at their local timeline.