Hi..
Welcome to HEALTHCARE MAGIC..
I have gone through your query and can understand your concerns..
As per your complain sore milky white tongue seems to be due to  Oral Yeast infection known as 
Oral Thrush while the  stuffing of nose and coughing up of phlegm and congestion can be due to 
Sinus infection causing 
Sinusitis..
You should consult an Otolaryngologist and get a clinical examination done and a PNS View of sinuses and MRI Scan can help in confirming the cause as well as clinical evaluation and microscopic examination of sinuses can help in diagnosis and treatment can be planned accordingly .
In case of Sinusitis you can be advised to take antibiotics like Augmentin, painkillers like Ibuprofen, saline 
nasal spray, steam inhalation and  steroid nasal spray..
Use a humidifier in your room..
For white coating you can gargle with Nystatin mouthwash and suck 
clotrimazole lozenges..
Take more of Probiotics like Yoghurt and Garlic..
Hope this information helps..
Thanks and regards.
Dr.Honey Nandwani Arora.