being an Se dom and Sx/So (and given your personality on the forum) it makes a lot of sense you would mistype as 7w8 (even now, I don't see much outwardly 6 about you, it must be much more internal)
His defensiveness and fire. Look at Marm and me, we both have it too. Sx and 6 do that together, apparently.
I think I may be a 6w7 instead of 4w3. Yup. It kinda sucks when I consider how much I've written in e-gram threads about the 4w3 qualities and problems and <insert Charlie Brown telephone> whawhawah.

I have been right there too. Embarrassing, but kind of exciting when you realize things are clicking into place when they didn't quite before.
Have you previously mistyped yourself?
Yes, as an INFP in the MBTI system and as a 7w6 and 3w4/4w3 on the enneagram.
How long did this mistyping occur?
I thought I was INFP from the time I took my first informal MBTI in like 8th grade to my 2nd year of college, when I finally bothered to learn more about the system and discovered myself as an ENFP.
As for the enneagram, maybe a span of several months floating in type limbo - it was always easier to identify what I am
not (1, 2, 5, 8, 9) than what I am. I'm still not 100% confident that I'm a w7 over w5. I definitely have some 5 in me.
For what it's worth, I think I also just recently realized that I might be tritype 648 instead of 649, like I had initially thought. But I have to delve much more into that to decide.
Why do you think you mistyped?
Enneatype is often hard to identify, I think because it's something the ego tries to "hide" from one to protect oneself. It's essentially defining your defense mechanisms, and those won't be as iron-clad if you can readily break them down theoretically. That's my preliminary thought on it, anyway. And it's just not something you really want to think about - the way in which you're constantly screwing yourself over.
As for me personally, 6 was a tricky conclusion. I didn't see it at first... I didn't even consider it at first. I didn't think of myself as a "loyalist" and I didn't associate with the hard-working, responsible, dutiful, suspicious type. That was in part because I didn't understand the "core" of the type, which is instability... and part because I was blind to my own traits because I'd only been in certain situations. I've been in my current job for a year now, and I have seen the hard-working and responsible and loyal side of myself emerge: hard-working because I like the certainty of everything being done and done well; responsible because I have a strong innate understanding of how I screw over the group and thereby myself if I don't pull my own weight; dutiful because I want to be good; loyal because I am idealistic; suspicious because I am anxious about losing my job or good schedule or position of respect.
Are you confidant in your typing now?
I am very confident that I am a 6 and sexual-first. In fact, I'm more sure I'm sexual-first than 6. I am moderately assured that I am a 6w7 and sx/so.
One trait that did help to clue me in to 6 was my indecision - not because of 9ish complacency, but because of 6ish overanalysis. That overanalysis is a pretty solid clue, too - 6 is totally, totally about overthinking. Way too much mental processing. Defensiveness was another - if you look at my posts, they generally tend towards the analytic side, I feel (good indication of a head type), but I would launch into all-out Te attack mode when threatened. I am definitely a reactive type - things in my external world change me internally more than vice versa. That's the 4, 6, 8 group - we use our mental energy primarily to respond, moment-to-moment, to the external world - whereas the 2, 7, 9 group tries to harness their mental energy to create positivity while the 1, 3, 5 group tries to employ mental energy become competent in specific areas.
Really, for me, 3 was out of the question because I have never been that much of an
external action person. I am much more of a thinker first, and I derive more satisfaction and stability from my thoughts than from my actions. I
do seek external validation, but it's not my primary tactic to protect myself - retreating into my head is. And 3, 6, and 9 are all technically approval-seeking types, but 3 seeks it for validation; 6 for protection; 9 for harmony. I have strong innate internal confidence in my self as a person and do not seek that externally, which makes 3 unlikely. 4 is probably the most likely second, but again, I'm more of a thinker than a feeler, in terms of the enneagram. If I have a problem, I don't tend to amplify emotions to sort it out - I tend to dive into analysis. That was really confusing because as an ENFP, Fi aux, I do have the "drowning in emotion" syndrome and as Ne-Fi and a very visual person I do have the tendency to cultivate certain emotional atmospheres for the sake of aesthetics, but I do not associate so much with focusing on the idea that something is fundamentally wrong with me (isn't something fundamentally wrong with everyone?). I also associate more with the 6's levels of health and deterioration.