Yes, it's all gonna be there in Magic Ring. You will be able to change timelines as you unlock them and go back in time (Note that if you played game till Autumn of 3rd year, you will not be able to go past that date).
So Autum of 3rd year is something like a point of no return? And if we complete the game and end a timeline, then ryoko resets it all the way to the start? or we can keep playing to see all the timelines? For example, we do the Riku timeline, it ends, and then we can keep playing on that timeline to see stuff or Ryoko locks it off and we go back to the start of the game?
No, what I meant is that you will not be able to go in future. That is, let's say you played the game until 10th Spring of 4th year, you can go back at any point before that time but never in 11th Spring Year 4, Summer Year 4, etc.
If you complete the game depending on a few factors you can still continue playing trying to complete all side quests. Those timeline with all lovers are not there just those lovers will be available after doing a quest. You will be able to choose which lover you want after that quest. Everything else will remain same.