3. My tenant has failed to pay or allegedly 'deducted' the rent for several months by the excuse that he suffered from minor water leakage problems or discomfort/disturbances. Can he/she do so and is that a good defence to the recovery of the payable rent/forfeiture?
Shortly speaking, the issue generally depends on whether the obligation to pay rent was dependent on the fulfillment of any obligations on the part of the landlord (e.g. repair or quiet enjoyment, if so agreed) and/or whether the tenancy agreement expressly allowed the tenant to 'deduct' any rent payable by any reason.
In most situations and in the absence of any special clauses under the tenancy agreement, the tenant's obligation to pay rent is independent of other obligations to be performed by the landlord. Simply put, no 'rent' is likely to become 'deductible' or 'set-off' as such even if the allegation of the tenant may appear to be true.
This is to say, the complaints by a tenant over the standard/quality/condition of living in the property is unlikely to constitute a sound legal defence to non-payment of rent. As explained, non-payment of rent alone would enable the landlord to exercise his/her right to claim for outstanding rent and even forfeit the tenancy (subject to relief against forfeiture exercisable by the tenant).
The above is only a preliminary analysis of general propositions of the law and whether such principle is applicable to all cases would be heavily dependent on the terms of the tenancy agreement and exact individual circumstances. If you encounter such subject matter, you are definitely recommended to seek assistance from legal professionals.