Goldfinch is hard to know sometimes by the internet you know, IMO the coop kid did the right thing but it sometimes work, sometimes it doesn't. It depends a lot of the rim, spokes, lacing and how good the wheelbuilder is you know. No idea what wheel you have in there either so hard to know whats going on.

CNY indicates that threadlocker is not the solution and I agree but since it works with stubborn losing all the time spokes then why not using it I do agree with him in his post but my suggestion about "gluing" the nipples was just to that thing fixed quick, actually the coop kid will add linseed oil that is basically the same thing than adding threadlocker the oil will dry and lock the nipples In my country some guys where using water to get the nipples to rust tiny bit.

As for the rim question, well there is no way to know even with brand new rims if they come with a wooble, you use the spokes tension to get the rim right but sometimes rims are so f... up that you get sections as cny is describing, lose and one side and super tight at the opposite side. Obviously you get a loooot more chances to get a busted rim when the rim is used or when the wheel is used.

I fix the wheels for a friend that flip bikes and sometimes and I have seen rims so bad that you have to hammer them to get the rim in the right direction so the spokes get at least close to the right tension in the lose areas, by the way is not uncommon to do that even with new rims, specially when using vintage NOS rims, modern rims are machined and that helps a lot to the rims roundness (but sometimes you get lucky to get the ugly duck too). Since you got a used wheel who knows what's the story behind it. Since I do wheels (not professionally as many builders in the forum) pretty much I do not believe the wheel is right unless I retouch them myself spoke by spoke, for the record even with new wheels is not the same to get a super expensive one than a wheel coming from a walmart bike even if its new.

Tension, this is a really delicate subject because you can snap a spoke because of too much tension or because of lack of it. you have to add too the fact that some people likes the tension in the spokes really high and other guys likes them right in the spot with the specs, since I dont use a tension meter i do this by feel using "the force" ... .

Learning to build wheels is always a good skill you know, maybe you can use this wheel to learn, it is a process but is not hard to learn quick, you will save some cash aswell because LBS now a days dont waste time with wheels, cheaper get news ones.

Hope this helps.