Unfortunately, given what we know, it's impossible to give good answers to all of your questions. Because the science of doping is unsettled, we don't know what role EPO would play in a scientifically based doping protocol for someone like Kiprop. And because athletics hasn't had a Lance Armstrong, we don't know for sure what protocols dopers are using.
To complicate matters, we don't know the basics of Kiprop's bust. We don't know what the evidence is, or where he was in his racing, training or doping cycle. We don't even know what country he was in when he was tested! Yet any of these could affect the answers to your questions.
If the reports are right and he failed an out-of-competition test in late 2017, it seems most likely was using EPO as part of his training routine. The idea is simple -- by boosting endurance, EPO allows athletes to train harder. This is the rationale behind altitude training, since lower oxygen levels boost EPO production.
As to whether using EPO during the off-season is less risky than during the season, again, it depends. Looking at the USADA statistics suggests that isn't the case in the US. USADA usually conducts two or three times more out-of-competition tests than in-competition tests, and tests a lot during the off-season. Since winners and high-profile runners get tested most often in-competition, you'd think someone like Kiprop would get tested a lot during the season.
Who know what the calculation is like for Kenya.