For your first question, you can get the configuration section "arena1" and get all of the values as a Map without any warnings. To do this, use:
config.getConfigurationSection("arenas.arena1").getValues(false);
Alternatively, you can just put @SupressWarnings("unchecked")
over the method where you're using that code. Since you know the type you're getting will be a Map, the warning doesn't really mean much, though some developers might consider this bad practice.
For your second question, you can use a similar method. getValues() is essentially a getAll() type of method, it gets a map of all of the keys and values in the section. So you could use:
config.getConfigurationSection("arenas").getValues(false);