You should know that a destabilized Somalia is Ethiopia's top priority. If Somalia makes a comeback, it will quickly try to annex Ogaden which has a lot of oil. So weakening Somalia by cutting the Shabelle river does not sound impossible in all honesty.
The Shabelle river makes no difference to the stability of Somalia. If a dam does not get constructed it doesn't mean all internal conflict in Somalia will come to a halt.
First it's ha-ha Ethiopia is landlocked, second it's oh no they can't use what they have in-land too........
No point predicting the future.