undefined> Can lemmy.ml open signups so they don’t need to be approved?

That probably won't happen - any old timers here correct me if any of the following is incorrect, I hope im not being confidently incorrect. The amount of resources that the server needs, and the amount of mods it would take for that, would be quite a bit. Think of it this way - its not like lemmy.ml is meant to be reddit as a whole. Its mean to be one singular small-ish instance of something similar, of which the server is ran on some private place by people largely doing it out of the goodness of their own heart. If one instance gets too big, that requires a lot of resources, and could likely kill the instance. Not only that, but open registration just screams bot abuse

Its better to have things more decentralized, and have people go to new instances so that theres not a huge influx that burns moderators and resources out. Im personally hoping that more instances pop up for more niche things - ie, a server for sports, or for pc building etc

communities on other servers aren’t automatically and easily available.

You may be aware but for others, on the main page you can change which communites are displayed. At the top, there is the option or Subscribed, Local, and All that's only a single click/tap. (I do personally think the default should be All)

• Subscribed shows you just the content from the communities you are following from any instance.
• Local shows you the content from the communities only from the instance you're currently viewing.
• All shows you posts from the communities of all the instances that are federated with the instance you're viewing from.

From the list of posts, you can then select a community from a different instance (these will have the community name with something like @example.ml at the end) which will bring you to that instance. From there, you can select the "Communities" option from the menu and it will show you all the communities that are hosted on that instance.

First of all, welcome! I hope you have a nice stay :D

Now, to answer your points:

1. I'm quite positive that won't happen any time soon because registration needing approval was the only measure that realistically stopped bots, and with the new influx of users, it would be a bad idea to turn it off. A solution would be to get more people to approve users, instead of disabling the protection. Either way, the developers are making an effort to push people towards other generic instances, because lemmy.ml is not supposed to be a flagship instance, despite always being seen as one (and understandably so).
2. Servers can't know what communities exist on servers they do not know, but I agree this should be more robust and easy for the end user. Federating servers should periodically exchange a list of communities to help with discoverability. For now though, there's https://browser.feddit.de.
3. Definitely. It should be made very clear. If you can, you should try opening an issue on the lemmy-ui repository about it, if there's none already.