Allow me to stick an oar in here.
1. Using the LAMA map, it's a little under 400 straight-line miles from the southern edge of the Forest to the Seridahn. Which coincidently is just north of Thesmar which might not be the best place to try to cross the river.
2. It's a little under 500 miles if he goes through Cheryk, From Cheryk he has two roads to the river, to Somyr and to Yairdyn which presumably have bridges or established ferries.
3. Hanth apparently is somewhere up towards Evertyn on the west bank of the river and presumably is moving supplies up the river which will be patrolled and defended.
4. We don't know how much Hanth knows other than a honking big Army went west to the retake Ft Tairys.
5. We don't know how much Ahlverez knows about Hanth's activities.
I'm not optimistic Ahlverez is going to get his army back to Dohlar because he's got that big, heavily trafficked and patrolled river to cross.
It's not likely A will be able to sneak up on Hanth because of the supply line patrols.
It's highly unlikely, IMHO, A is carrying bridging material for his engineers to put a bridge across the river.
There may be bridges at the Somyr-Cheryk rd crossing and at Yairdyn. The bridges will be guarded by relatively small entrenched forces which will have to be removed before A can move his troops across across the bridges. Any attempts to take the bridges by assaults will be costly and time-consuming which will allow Hanth to move against him.
So, unless A has come across a large deposit of handwavium it doesn't look like he's gonna get his men across that river.