Baab, you're welcome

your summary is correct, the first ん is like ŋ and the second is like ŋ or m.
Flaminius thought the second IS nasal vowel but it's not correct, he and SoLaTiDoberman talked about nasal vowel in the Japanese texts.
1番目の「ん」も'ng'式に発音しておいて、その直後に「ば」のbを発音するために口を閉じる、というやり方です。
SoLaTiDoberman explained, in his post #7, that you can pronounce the first ん like ng and then meet your lips to pronounce b sound, and be that as it may he pronounces the first ん like m, and every time I pronounce the first ん I pronounce it like ng, I think we have two options.
But as for the second n, though both of ng and m are correct, it's far from nasal vowel. I can't put my finger on why he thought "わたしは英語では/ŋ/は単語の終わりにしかこないので良く分からないなとおもった(In English, ŋ sound is always at the end of words) " and my students in English class sometimes say this. When you speak English everyday or on a regular basis you know ng sound appears in the middle of words, e.g. singer, ringer(they're diffrent from 'finger' in which n is pronounced as ŋɡ) kingdom, winglet and too many to count. Point is, both of two ん in こんばんは are like ng in the middle of words, I speak French on a regular basis, nasal vowel is in French, from my firsthand experiences in these languages, the second ん is nowhere near it and if you're told that the second one is a nasal vowel you could be confused, that's why I think, though I've never heard people pronounce the second n like nasal vowel some people might utter it like that, nasal vowel is out of place.
Last but not least, Baab knows ん has several variations of sounds in the first place, and I explain ん is LIKE ng, if you're aware of that it has variations, you can easily tweak pronunciations yourself to make it sounds like the exact ん sound, I explained it comparing it with English sounds since I think that's handy for every learner.
So, it's good to give a shot with the back of your tongue against the soft palate
Spu