>As for why "neun...und sechzig" is ambiguous: with that weird pause it's not clear if this is still 69 or literally "9 and 60" (which is what the word "neunundsechzig" means), i.e. 9-60.
9 und 60 vs. 9 hundert 60, if you speak quickly then the "ert" part of hundert is often swallowed, making it more difficult to discern between the remaining "hund" and "und".
I'm assuming you're a native German speaker too? Must be a regional accent thing, then.
The closest I can get to ambiguity by slurring "neunhundertsechzig" is "neun oder sechzig" or "neun, neununsechzig", never "neunundsechzig", and I have to make an effort to speak faster than I normally would even if I was reading out numbers in a hurry. I've also never run into this ("hundert" and "und" being mixed up) in my daily life, so I'm surprised to hear that this is something you hear often.
9 und 60 vs. 9 hundert 60, if you speak quickly then the "ert" part of hundert is often swallowed, making it more difficult to discern between the remaining "hund" and "und".