Both «نَسْتَعِينُ» and «الْعَالَمِينَ» end in nun after madd ya, but the final vowels differ. «نَسْتَعِينُ» has dammah on the nun; «الْعَالَمِينَ» has kasrah. That small phonetic difference affects which stopping options are available.
Nasta‘in: ordinary iskan, correctly performed rawm, and ishmam are distinct permitted possibilities where their conditions hold.
Al-‘alamin: ordinary iskan and rawm may apply, but a final kasrah does not allow ishmam.
If the final vowel is removed completely, the nun becomes still and the preceding madd ya may acquire the condition for incidental madd. However, a later qualification affects madd when rawm is used. The final vowel and the duration of madd are related issues, but are not identical.