The IVs will still vary based on the inheritance of the parents for that PID frame. different PID frames have different inheritance and so there is slight variation in possible IVs. If you hit your seed though, you will have at least 3 flawless IVs, but usually you will end up with 4 or 5 flawless IVs.
Be fast, be consistent, and listen to the pitches. That's all there is to it really. Eggs are like wondercards in that you are just manipulating the PIDRNG so all you are doing is flipping birds around, nothing fancy. The only possible issues that can arise when RNGing eggs is 1) wrong Timer0 and 2) that one school teacher with a PhD in trolling. As long as you are fast enough though, she won't move and everything should go fine. As for how many frames she may advance, that varies, but for me it is a solid 2 each and every time. This can easily be handled with pitches though as you will see if she moved or not and what your SSF is. Also, the 2 pokemon don't affect the PIDRNg, so ignore them.
Well, seed searching takes longer, but other than that, no. the first thing I ever RNGed was a shiny flawless egg, so if someone can learn RNG by breeding, it can't be too difficult. As for specil cases dealing with genderless pokemon, Ditto is always considered the female parent for the IVs. that's about all that is different.
You could just be having a horrible day with that teacher. Try listening to the pitches as you advance and adjust accordingly if need be. With enough persistence you will get it eventually.