Yeah, I think we're at an essential misunderstanding.
I don't mean for these gifts to be matters of official protocol, but elements of the basic socializing that high society dorfs would do. They're not required to have any friends, so long as their obligations to the Mountainhomes are met, but if you want to foster relations with someone, that's not done by just sitting in a cave.
You're right in that a shiny bauble wouldn't, in and of itself, prevent a nefarious king from invading. But what happens if he's looking to invade someone, not necessarily you, and he finds that three of his dukes actually think you're a nice guy, and you gave his wife those marvelous shoes last year? He wouldn't reject the idea of attacking you, but he might not be as eager.
Climbing noble ranks directly through gifts is right out. However, finding that the Mountainhomes are requesting 30 Pig Iron Bars this year when you have no flux stone could be a pain in the arse...but having an existing good relationship with another fort that does might get them to fill the order for you.
Miitary strength would dictate how hostile civilizations would treat you, but that just prevents you from getting invaded. How about getting your fortress *respected*? Wealth alone won't do it; that's the nouveau riche problem. Military might is often scorned by the multi-generational noble houses, who don't like getting their hands dirty. No, you have to get respect for your newly-elevated nobles by earning it, and that will be harder.