(1) To get the fastest time around these turns, I suggest staying as close to the inside of the track as possible while at the same time doing some mini turbos. You should be able to make at least 4 mini turbos.
(2) You should be able to fit one mini turbo in right at the beginning of the tunnel as it turns in. At the end of the tunnel, if you have a Turbo Mushroom, use it just as the track starts to slope up. If you don't have a Mushroom, you could try to make another mini turbo on the turn.
(3) In either Mario GP or VS mode, you will find a item box attached to the bottom of a balloon. As it comes down to ground level, do a little jump by pressing "R" on the controller. Inside the box, you'll be happy to find a Spiny Shell!