1. When placed adjacent to a Haven the stability gain is only +5 when, if I understand correctly, it should be +10 (for the farmer and market quarters of the Haven). 

EDIT: actually it seems it's only the tooltip values that are not correct, once built, it actually provides the +10 stability.

2. When the civic "public happiness" is chosen the cost of the triumphal arch does not change. Since it's supposed to count as a common quarter, its cost should then be reduced.