Hello all....This a question for the experts. :D If I have a grafted seedling which has already grown BUT has not yet fruited, would it still be effective if I obtained scions from this grafted seedling's branches to graft to other seedlings? In other words, would this be as good as obtaining scion from the mother tree?

thanks in advance.

No need to be an expert to answer YES.

thanks sylvain! Anyone know if this applies to most other trees such as avocados, durian, and other tropical trees? thanks in advance.

yes

If you want to get a fruiting plant of a variety as quickly as possible, get budwood from the mother tree.
If you cant do that, budwood from a mature seedling,as you say, will get you there, but maybe not quite as fast.
All that said, it depends on the cultivar too, you have to be sure your seedling is highly likely to be true to type, some seedlings of some varieties wont be true, and even then, "true" seedlings might show slight variations.
Seedlings may tend to continue to show juvenile characteristics when budded onto stocks, ie thornyness and vigorous shoot growth, compared to the original budline.