LJ,
I'll give a stab at this. I don't know if the 5th gear being loose will make it pop out of gear but I do know that if the gear is loose it is hard to go into 5th gear. The shop manual says that the transmission will slip out of gear if "misalignment or damaged shift components or worn teeth on the mainshaft gears or synchro components are evident. " I believe also if the syncro struts and springs are not working properly, the transmission will slip out of gear.
The noise from the 5 gear is because of the wear on the transmission and what you hear is all the worn clearances on the parts. (did that make sense?) I just rebuilt mine and 5th gear noise would drive you crazy. After the rebuild, no more noise. Hope this helps.
WD