The Fairy Bow, Slingshot, Bombchus, and Fishing Rod items behave differently from other items when set to the B button.
When these items are normally placed on B, the current B item is tucked away into a temporary spot in memory (or Temp B for short) for the duration of the minigame, and when the minigame ends, the Temp B item is returned back to the B button.
However, the memory used for Temp B is also used for something else outside of minigames: It stores whether or not the item on B can be used, and whether the B button should be dimmed out or not. This memory value is set to FF whenever the B button is to be disabled (like when swimming in deep water, or climbing a ladder), and 0 whenever it is re-enabled.
Attempting to put the Fairy Bow, Slingshot, or Bombchus on B outside of playing the proper minigame (with Bottle Adventure or cheat codes), will cause the game to push the Temp B item to B until they are no longer on B. The Fishing Rod behaves slightly different: You can keep it on B outside of the minigame, but if you pause the game, you'll push the Temp B item to B.
This explains why you can convert the Fishing Rod into Deku Sticks. When you first steal the rod, if you pause the game here your previous B item will be restored. If you jump into the lake, FF will be put in the Temp B slot, dimming the B button. Pausing before standing on land will put the FF item (the blank item) on the B button. If you stand out on land and then pause, the Fishing Rod will turn into a Deku Stick, since undimming the B button puts 00 (Deku Stick) into the temp B slot.
Typically, the Temp B slot is set to 0, since this is the value the game set whenever the B button should be re-enabled. As a result, attempting to put the Bow, Fairy Slingshot, or Bombchus on B with Bottle Adventure usually puts the Deku Stick on B instead. However, you can easily put the Kokiri Sword on B instead.
Just set up Bottle Adventure so that you'll put either a Bow, Slingshot, or Chus on B. The easiest way is to put a fish on C-Right as a child, and then have either 3, 6, or 9 Deku Nuts in your inventory. Then play Bombchu Bowling as a child to put the Kokiri Sword into the Temp B slot before going back to the future.
Alternatively, you can go Back in Time with Fishing Rod on B as Adult Link, play Slingshot or Bombchu Bowling to store Kokiri sword to Temp B, then return to Adult Link and pause to get Kokiri sword on B.
Before you obtain Epona, you can pay 10 rupees to ride the horses at Lon Lon Ranch. If you leave the ranch before your time runs up with Farore's Wind on B, the game will get stuck in a bizarre state that makes it possible to keep Bombchus and the Slingshot on B, by playing the proper minigame as a child. However, like the Fishing Rod if you pause the game you will push whatever item is on Temp B to the B button. Note that the Fairy Bow as adult does not work.
Two side effects of this state are that if you dim the B button, the game will not undim the B button automatically like before. Secondly, all C-items other than the Ocarina are unusable. While the ocarina can be played, the effects of all the songs will be non-functional.
To restore the game back to normal, simply return to Lon Lon Ranch during the daytime.
Although you can store Bombchus to Temp B through Kiddy Capers, it is impossible to use BA to push Bombchus to B as Adult. Leaving Lon Lon Ranch requires having Farore's Wind on B, so you would have to dupe another bottle onto B. However, leaving Lon Lon Ranch disables dropping a fish/bugs, so it's impossible to dupe a bottle over B.
However, you can wrong warp to the child version of Castle Town, allowing you to enter the Bombchu Alley and obtain Bombchus.
If you have either the Slingshot or Bombchus on B, you can pause without losing either of them. Simply play the fishing minigame to push the Slingshot or Bombchu to the Temp B slot, then steal the rod. Once outside, simply pause the game to put the Slingshot or Bombchus back on B. As long as the B button does not dim, you can keep pausing the game. This trick has little use as a child, since the fishing minigame is surrounded by water, but as an Adult the water is shallow enough before beating the Water Temple that you can avoid entering it.
If Windy B is active, Minigame items do not turn into Deku Sticks when BA'd. This is not as simple as it sounds, however, because you need Farore's Wind on B to activate Windy B, and Farore's Wind on B does not work for bottle adventure. You need a bottle item or trade item on B to perform Bottle Adventure, so the only way to do both is to do the following steps:
If you perform the Gerudo Archery game wrong Warp using RCE (Remote Control Epona) while you have a Fairy Bow on your B button, it can stay on your B button after the wrong warp.
It is possible to obtain Bow on B without Bottle Adventure by performing RCE while sword is equipped and you have something in your Bow slot such that Bow is your active B button upon entering the fishing pond. This is most easily performed using death fairy revival RCE as shown in the below video.