Good questions.
Since Lily is small her normal movement is 25' per round. So she can move 25' and perform any one action. So, in this case - 25' and then fire a sling stone, for example.
Talking does not use up an action (usually it isn't more than just adding color to the post anyway.). Unless you are specifically attempting to convince some about something or explain something super complicated. Generally no - it would take a really extreme situation for me to say it would be your action. So assume that you can just babble away as much as you want.
You get one action per round unless some condition changes that. So assume one. Typically you would just state that one action. However, given that we are playing by post and not sitting around a table where we can talk and adjust on the fly, there is some flexibility. If you wanted you could post your basic plan and then after that add a note that says something like, "If when it is Lily's turn the goblins have actually done X or all of them are dead, then Lily the Wondrous will do Y." Generally keep it to just one turns worth of action (So not, target this one this time, then that one next time, then run up the hill, then aim there.) But a couple of clariflying comments at the end of your post is perfectly reasonable. People often put OOC: (Out-of-Character) in front of such clarifying bits. Rereading your post - yes, I think I am describing something like what you called a primary and secondary action. It is just that since you only get one action per round the secondary action only happens if your first does not for some reason. Also, you do get one "item interaction" per round as well - things like opening a door, picking up an item or whatever. Not that Lily would ever need to do things with items.
I roll initiative for the characters and keep track of it. There is a definite order in which things occur. It is not determined by the order in which players post on the thread - it is the initiative order of the round. I know it, you don't. However, if you look back at previous fight sequences you will find that I generally always had characters acting in a certain order in my posts. So you can figure it out if you want. I'm not hiding it and will tell you if you ask - around the table we would all have access to this information. But since in real life such information isn't know I don't post it here. Initiative order is determined by a dice roll and will change for each conflict.