What if Goku Accepted Frieza's Job Offer on Namek?

He might eventually have no choice but to challenge Freeza again. If he joined in hopes of finding strong opponents, he'll probably be disappointed when he finds out that 99.9% of aliens are no match for his base let alone Kaioken. Freeza and his family members would be the only ones who could put up a fight against him.

If he joined to protect his family and hope that Freeza would spare the Earth, it wouldn't take him that long to surpass Freeza. I like to think that his base was at its limit after his last Zenkai on Namek so the only way to get stronger would be to push himself until he unlocks SSJ. If he can still get stronger in base, he would only have to double his power for his Kaioken x20 to match 100% Freeza. Freeza could probably maintain his 100% form longer than Goku could maintain his Kaioken x20 so Goku might have to strengthen his base further so that his Kaioken surpasses Freeza. I still think this could be an interesting universe to see how they would interact for the time until Goku surpasses him.

EDIT
This could be an alternate universe to RoF where Freeza decided to train those four months long before he went to Namek. Goku turns SSJ and Freeza easily defeats him in his 2nd or 3rd form and offers him the job again . Regardless of how much training Goku does, he could never surpass Freeza without the SSJ God ritual or Whis's training so Goku might have no choice but to join him in order to protect the Earth.

he would be getting paid more than a farmer's salary.

I think what would happen in this scenario is that #17 and #18 terrorize the Earth until they're killed by Dabura once Babidi shows up seven years later. There wouldn't be enough energy to release Buu since most of Earth's population would be extinct and no strong Z fighters left. Cell would be born several years after that because there wouldn't be a time machine. If Dabura didn't kill the cyborgs, they might kill Cell once he tries to absorb them since he would be much weaker without Freeza and King Cold's DNA. Dabura would probably the strongest opponent Goku and Freeza could face unless Babidi could release Buu or Cell leaves the Earth and absorbs enough alien lifeforms.

Which begs to the question... WTF happened with the Android problem in U7 (Gast)?

I can't see that happening in any universe. Goku doesn't have any evil in him at all and would never turn to the dark side that far into his life. The only evil version of him in multiverse was the one who didn't get hit on the head as a baby and retained his aggressive tendencies. Saiyans aren't born evil, they just have an inherent disregard for certain values that build up morality.

I could see Frieza kidnapping and brainwashing Goku, or wishing for him to become a loyal servant via the Dragon Balls, but the outcome would probably be Frieza killing Goku once he becomes a Super Saiyan (or trying to kill him and failing, or blowing up whatever planet he's on, or killing him once he shows signs of becoming a Super Saiyan).