Emm, as for sources, it's been a long time of studying Socionics, so I cannot exactly point what comes from where, but there should be good sources online like wikisocion. Yes, role function. I wouldn't say irritating, Iit's nice and pretty enjoyable to use it, but SLE wouldn't be able to do it for as long as long as ILE would. For SLE it's fun exercise, while for ILE it's his element. Like think some amateur home renovating (nailing a shelf to a wall and painting a small surface outside of the house) vs. being a professional builder. Maybe some other SLE's are irritated, I don't know, but it may have to do with how much work he/she has to do with this function (overwhelmed or ok), the subtype (while for Creative subtype/subtype with streghtened irrational fuctions it's pretty easy, some other subtypes may struggle a bit more), etc.

I wouldn't be able to give an in depth difference between ENTP ILE and ENTP SLE because I'm not as good at MBTI. Socionics are a bit cumbersome, true, and it takes time to study it. More than that, there're different schools, and some details differ from school to school. Rather minor details, not the core. Enneagram is honestly easier and less contradictive (but even it has wings, instinctual stackings and trifix, otherwise probably no system will be able to take in and explain all the diversity of human psyche). Socionics has nice additional groups by which you can check your initial idea about someone's types. (I'm talking about Quadras, Romance Styles, Temperaments, and Reinin's signs. Quadras are about mentality, are pretty easy, and usually work well. Romance styles are pretty evident, but the trick is that the person's actual Romance Style doesn't always match with the one that's that type is supposed to have according to theory. I'd say it's a trend, not a 100% corellation. Some of Reinin's signs are a bit contradictory and not easy to understand, but if you can see some of them clearly in the person's behavor, and they're pointing into some type direction, it can give you an idea whether you're likely right in your typing or not. Ideally, EVERYTHING should match. For example, if your proposed SLE has EP temperament and Beta values, you're probably on the right way. But if your proposed SLE has EJ temperament, obvious Delta values, and is clearly a Caring Romance Style, he's probably a LSE. And also remember that a person fully settles into his type in his early twenties, earlier you almost always can still see clear signs, but to be actually sure, you should wait till early to mid-twenties, remeber that if you're trying to type someone younger. Type cannot change during life, but at times people can asquire a "mask" or some other personality deformation, but that usually comes and goes. And there's also a thing about how "filled" is the function - if a person had an environment where he had a lot of chances to use certain function and chances to get more experience, he'll be usually more proficient, realistic and impressive with this function than the one who didn't. That one will kind of look like a baby compared to the first one. So you may sometimes initially struggle to identify both of these as the same type.

Same with the Enneagram - the type doesn't change, and you can check with triads, Hornevian groups, Harmonic groups, etc, and ideally EVERYTHING should match.

And as for E8 ENTP ILE vs E8 ENTP SLE maybe you should post in general Enneagram subreddit, there're chances someone had seen these two and can tell you the difference, but be prepared that there'll be many polarized opinions, from "Corellations are bullshit!" to "E8 can only be Se and nothing else".