The Secret and Ring Passwords in the game are based on the name of the file. If you are using a password from someone else's game, that password will not work because the name is different. If a side quest password doesn't work, make sure you're using it in the proper linked or original game with the same file name as the game you received the password from.
The maximum is 16. You start with four, then you can collect eight Heart Containers from dungeons, three from collecting Pieces of Heart, and one from a side quest. In a normal non-linked game the maximum number of Heart Containers is 15.
If you purchase the Strange Flute from the Shop before heading to Spool Swamp for the first time, you will get Moosh's Flute instead of Ricky's. If you win Dimitri's Flute in the Subrosian Dance Hall before heading to the Spool Swamp for the first time, Dimitri will be your pet. If you're playing a Linked Game version, you will only be able to find the pet which you received in the first game. For example, if you received Moosh in Oracle of Ages, you can only get Moosh's Flute in a Linked Game version of Oracle of Seasons.
If you use a password to start your game, your name is chosen based on that password.
No, but to collect everything and see the true ending you will have to play through both games.
The Link statues signify that you have completed the game.
Try using a Gale Seed to warp to a Seed Tree near the Maku tree.
You cannot enter the Maku Tree until you've completed Level 2: Snake's Remains. Once it has been completed a door will appear on the Maku Tree.