Re: Jon Snow's true parentage...
Yes, I admit that I've no proof for this Guardian Angel theory. I accept that on the surface Edd, Jeor, Qhorin, and Donal all appear to have purer and nobler reasons for serving in the Night's Watch than Chett. I don't know that age is a reason for being more or less susceptible to bribery. Young men can be full of zeal like Lancel or frustrated like Pate. Old men can be faithful like Ser Cortney Penrose or weak willed like Pycelle.
As for my proposed guardians...
I questioned Qhorin's origins earlier. His name sounds like he could be from the Free Cities and not from the Seven Kingdoms. He could have been planted years ago. In the movie No Way Out, this was the situation for the main character. Patriotism is a powerful motivation.
Aemon, Donal, Edd, and Jeor have no need for money, titles, or women since they are on the Wall. But payment can come in other forms.
What was eating the Old Bear? His son's future and livelihood... he confessed this at his death. Illyrio was in a position to help Jorah... and he did help Jorah by introducing him to Viserys and Dany. Jorah had the option of serving Robert through Varys or the Targaryens through Illyrio. Either way would have been proof from Illyrio to Jeor that Illyrio was faithful in paying for Jeor's services.
Donal Noye was a smith in service to House Baratheon. It appears he was the master smith either at Storm's End or Dragonstone where he personally knew Robert, Stannis, Renly and their parents. He seemed, like every one else, to have preferred Robert over Stannis. He went to the Wall when he could no longer perform personal tasks to his own high standards, iirc. What if Aemon or Illyrio whispered that Jon was not Eddard's son, but in fact one of Robert's many bastards? He might have been persuaded to help protect and groom Robert's son to carry on the proud Baratheon tradition.
Also, spies may not even know who their real master is. Durin the Cold War, the following scenario took place more than once. A French information analyst with Arab sympathies is approached by an Arab. The Arab says that Egypt is not privy to NATO plans while the US shares plans with Israel thus putting Egypt at a severe disadvantage. Money does not trade hands because the Frenchman has altruistic motives for passing on secret NATO information. The problem is that the Arab is not working for Egypt, rather he works for the Soviet Union. The Soviets have gained a free spy inside NATO. I'm not picking on France because this happened to the US, Great Britain, and many others.
Eddison Tollet, aka Dolorous Edd, hails from the Vale. House Tollet's loyalty is to House Arryn. Jon Arryn was the Kingmaker twenty years ago. He stood up to House Targaryen and unseated a 300+ year dynasty. He married Lysa Tully, a woman known to be fertile, so he could father a son in his later years. He made Robert marry Cersei to cement the Lannisters into his coalition. He personally went to Dorne to bring them under the Iron Throne. He was a planner. He did not leave things to chance. He could easily have sent a man as an agent to report from the Wall. He could have planted Edd as long as twenty years ago. But with Jon Arryn dead, who would Edd have reported to? Perhaps Ned stumbled across part of Arryn's old spy network and told Edd to protect Jon Snow. I'd say that Varys was at least aware of Arryn's spies... I think it might be likely that Varys co-opted or took over Arryn's network when Arryn died. Since Illyrio pulls Varys' strings... Illyrio is now Edd's boss... and since Arryn died before Jon went to the Wall Edd and would not be suspected as being a protector for a Targaryen. Edd could have gotten a message from Illyrio saying, "Arryn is dead. Protect Jon Snow. I am now running the spy network. Sincerely, Lysa Arryn." How is Edd gonna know the difference? He's stuck on the Wall. He's just staying faithful to House Arryn.
Perhaps Aemon thinks saving Jon is saving the Seven Kingdoms. Perhaps he is trying to make up for eighty years of not supporting his family. Perhaps he could not deny his Targaryen heritage any more.