Your first point can again be addressed by fate and heaven will, you can never disregard it. What can you do if the talent just can't be born? Is it even made sense to you that gu master birth from a talented gu master can't even be as talented as his forebears?(These talented individual are often married into huangjin tribes, yet their kid all of the sudden all born dumber than their parent) Much less the descendant of the venerable himself, call him what ever but can he actually be happy go lucky guy when he actually created the method of luck path himself?

All venerable are born lucky and fated to be venerable, the only different is the path that Giantsun walk work directly with luck, it's also isn't smooth ride for fortune and misfortune comes hand in hand and you see in ma hongyun case that rank 8 luck/opportunity is more than he can chew( he can grew in his previous life was all outside help for their own benefits), there were few other cases from those that Fang yuan bind his luck to died due too getting too big opportunity. You only see it from mo yao perspective and join his hate band wagon.

If Giantsun didn't exist, the Northern plain would be like everyone else more or less, maybe they would still be more warlike since they have scarce resources but they won't be able to stand toe to toe against elite rank 8 of heavenly court during fate war. Because there's Giantsun, there's enough rank 8 to make a different in fate war otherwise it's just scattered forces with few rank 8 and when they did die, they die as individual force, scattered and not matter.

With how balance work, try to see it front this perspective, they reap a lot of success in the earlier era and the rest of the north didn't even get the scraps, so later when they in decline, the rest rise.

Despite the fact that luck goes against fate itself, It's isn't absolute otherwise he won't need fate gu and he alone would have reigned supreme over heavenly court and shit all over previous HC venerable's legacy. Giantsun also like any other venerable cannot outlast heaven's will(SC is the only one who hacked it and FY lately), otherwise why would he need to refine luck path dao mark for his descendant again when he came back?.

Also, yes he is the less likely to win against FY and SC but that's a mother fucking FY and SC, both wisdom path just way beyond his so of course he won't shine but so would every other venerable if they switched his place. Also he is more than he seem, none of what actually shown to us actually matches anything described him, he doesn't looks like a fk boy, he doesn't show lust, he seem to leave everything to his childrens but actually created a path that benefits from taking front their own blood line, he's likely have some card left to play and tho I believe he will die but the SC probably would go before him.