What are the general causes of rhinitis and rhinorrhoea?
Pollen; overdose of local preparations; allergies( hay fever) and viral infections

Which drug groups can be used for the treatment of rhinorrhoea? Name examples from each group.

antihistamines - Promethazine

Corticosteroids - Prednisone

Mast stabilisers - Ketotifen

Mucolytics - Mesna

Antibiotics - Mupirocin

Diverse drugs - normal saline

How do the decongestants differ with respect to the mechanism of action and duration of action?  How are they administered typically?
Decongestants activate alpha-adrenergic receptors of the nasal mucosa. They oppose vasodilation which causes vasoconstriction causing reduction in nasal airway resistance.

They are usually nasal sprays - through the nose.

What is rhinitis medicamentosa?  How is it treated?
Rhinitis medicamentosa is a condition caused by nasal decongestants

The first step to treating this condition is to stop using nasal sprays. Imidazoline can be used to treat it.

How does the first and second generations of antihistamines differ with respect to the mechanisms according to which rhinitis and rhinorrhoea are relieved?  What are the advantages of the second generation of antihistamines?  Why should they not be used to relieve cold rhinitis?
2nd generation antihistamines do not block muscarinic receptors and are useful for long-term  treatment of allergic rhinitis.  Because histamine plays no part in cold rhinitis (but bradykinin does) these drugs do not help to clear up cold rhinitis.

When are corticosteroids, anti-allergic drugs, mesna and normal salt solution valid and how are they administered? 
For prophylactic allergic rhinitis. They are used topically which also minimizes side effects