Ulcerative colitis is known to have relapses and remissions despite being on maintenance therapy with Asacol and Azathioprine. The frequency of relapses can be reduced by such treatment, but cannot be completely prevented.
You have not mentioned your weight, and although the dose of Azathioprine that you are taking (100 mg) may be correct, the dose needs to be adjusted to your body weight. I usually prescribe Azathioprine in the dose of 2.0 - 2.5 mg/kg body weight to my patients with Ulcerative colitis, ensuring that they have 3 monthly blood tests as part of safety monitoring. This is the dose that has been shown to be effective in maintaining remission of the disease in large clinical trials in the Western World.
The maintenance dose of Asacol is 800 mg twice a day and you should be taking this dose. Taking Asacol thrice a day for maintenance has not been shown to be better than taking it once or twice a day. The twice a day dosage regime is better tolerated by most patients.
Surgery for Ulcerative colitis is very specialised and is reserved for patients who have been treated effectively by a Gastroenterologist over a period of time and then assessed to be suffering with medically refractory colitis. Surgery is also available for some other situations in ulcerative colitis such as acute fulminant colitis and toxic megacolon (which do not apply to your case). The hospitals I know of in Delhi that carry out such surgery are All India Institute of Medical Sciences, New Delhi and Sir Gangaram Hospital, New Delhi.
No other tests need to be carried out for managing your condition. A colonoscopy may be required rarely, once the diagnosis has been firmly made.
New treatments for Ulcerative colitis are in the experimental stage and not available outside of clinical trials yet. My advice to you is that you should be under the care of a Gastroenterologist with an interest in Inflammatory bowel Disease, and he/she will be the best person to manage your condition effectively.