Yes, Mangrove forest works ...

Yes, Mangrove forest works as the shield against Tsunami in coastal areas. A very big mangrove forest exists in Sunderbans (one of the largest of such forests in the world) area near the estuary of Ganga. However, Mangrove forest has its own ecosystem. Concern is whether this Mangrove ecosystem may work as an invasive ecosystem to the existing ecosystem if it can be planted in upper stream of estuary.

Channel deepening will really be benefitial  in this case. However, diversion of channel may not be possible. Let alone other reasons, getting extra land for diverting channel in this region will give rise to a number of legal issues.