How to activate picked-up items (rockets, boosts) in racing mode

As far as I know you can transfer only once. But you'd better try opening a ticket on Rockstar Support website to clarify this and/or ask them if they can help you with your request.

Before opening that ticket, make sure:

1) your xbox one and xbox 360 accounts with which you are trying to make a transfer are the same.

2) make sure, that xbox account id is linked with Rockstar Social Club account on: https://socialclub.rockstargames.com/settings/linkedaccounts

for more info please see: https://support.rockstargames.com/hc/en-us/articles/202892778-Grand-Theft-Auto-Online-Characters-and-Progression-Transfer-from-Xbox-360-or-PlayStation-3-to-PlayStation-4-Xbox-One-and-PC

No, you can not.

While you can transfer your character from Xbox 360 to Xbox One, you are making a copy of the character, and converting it to the newer console. From this point on, the Xbox One character is completely differant to the character on Xbox 360.

You also can not transfer your save back to the 360, as this feature was provided to allow players to upgrade consoles without worry about starting over, and does not support going in the opposite direction.