Does my bum look spotty in this?

When buying your underwear at the shop do you wash them first or do you wear them straight away because you may believe they are clean? I always advise washing your underwear first because many clothings are chemically coloured which may also cause skin irritation. When washing your cloth be aware about laundry detergent, if you are suffering with these skin problems already, it's important to make sure you look out for fragrance and dye-free detergents.
Diet is also very important when you suffer from spot's in the booty area. Its likely your diet causes inflammation in your body and digestive system, therefore I would advice to introduce more greens and still water since sparkling water is carbonated water 3-4 which means it's slightly acidic. Drinking enough water also helps to dilute toxins from your body, which helps a lot with your skin appearance. Drinking water is the cheapest and best way to start with a good skincare routine.

Try to incorporate green tea into your diet, it's rich in polyphenols (poly-what?) Polyphenols increase the blood-flow and oxygen to the skin, improving its overall look, feel and most importantly, health.
People who experience spots on their stomach often have high blood sugar levels. Sugar is just as bad for your skin and one of the main ways sugar has a bad effect on your skin is by insulin levels to spike, causing inflammation throughout the body.

Spots around your chest area can also indicate a food intolerance. 

Suffering from Candida  infection of the skin can cause symptoms like a rash on the skin folds, genitals, middle of the body. 
If you like to find out what your ph-level is you can go to the pharmacy and get some ph-strips and test your saliva once you get up in the morning. It's fun to check the colour chart to find out your alkaline.
Experiencing severe stress may also cause skin problems since this has been proven in clinical studies to worsen the overall skin condition. This is because stress induces the adrenal glands overproduction of cortisol , a steroid, which in turn makes sebaceous glands produce more oil and make skin extra oily. Thus the reason why stressful periods, people increase in acne get more inflamed, pus-filled papules.
My tip: I always recommend Magnesium to lower stress level, since magnesium is necessary for detox of cortisol. It affects the sympathetic nervous system and norepinephrine released, which causes cortisol release as part of your stress response.  

b) Are there any ingredients to avoid?

I would recommend to avoid Fragrance products,sulfates, phthalates, alcohol, PEG and paraben 

c) How do you recommend treating the skin?

It depends on what the skin condition is diagnosed, since we are treating the entire body and need to look at the whole picture, not just the skin , lifestyle and habits in order to suggest the right products or treatment to achieve the desired result/outcome for the client and create skin confidence.

In the clinic we would treat the skin with Ozone therapy and LED light, Biotherapy or Mesotherapy. 

I believe this kind of skin conditions need to be dealt with a professional skincare expert in order to treat the analysed skinconditon correctly and not to create a wishes circle hopping from product to product that can also cause more skin reactions therefore skin concerns should be improving rapidly.