1 Yellow Spots
I've never seen that before but it could be some sort of fungus. Do you have space to isolate your FRT to see if it improves there? You can try using a soft cloth and dip into and then wipe the shell and skin of the PNT to a certain extend that the dead skin and fungus can be wiped off.
If possible, you can dip the cloth into warm water as the warm water could kill off the fungus as well.
When you place your FRT back into the water, add some salt and increase your water temp as this helps to increase the immune system of your FRT.
Take your FRT out for a sun bath or UV bath. You do this by putting your FRT in a shallow basin and cover it with some water but leave the shell exposed. 15 - 20 mins will do.
2. Red Bumps
It could be abrasion and it's getting inflamed. I would replace the gravel with soft sand or just keep the tank bare.
3. Tank Mates
Can I know what are the tank mates of the FRT?
4. FRT Price
It's banned for sale where I am now, but in Malaysia which is like a 15 min walk across the bridge joining our 2 countries, I can buy a 3" FRT for RM$ 6 which works out to be a little over USD$ 1 :x