Written German has verbs like <können> ‘can’, but some (quick, lazy, …) speakers – myself included – pronounce this word form without any /e/ or schwa sound in the second nucleus. There are still two syllables, the usual IPA transcription [ˈkœnən] thus would be wrong (and so would be [ˈkœn̩ən] with U+0329 added to show the ambisyllabic status of the [n]). I’m not sure what would be the correct one: [ˈkœn̩], [ˈkœnn̩], [ˈkœn̩n̩], [ˈkœn̩ː]? It’s definitely not pronounced just [ˈkœn]! I even wondered whether it could better be described by a tonal marker, as in [ˈkœn↑].
It should be [ˈkœnn̩]. IPA notation hardly depends on what precedes. And since the first syllable remains closed (doesn't it?), there must be a pure [n] there. Besides, könn-, [kœn], is a distinct morpheme, there is no morpheme [kœ], so one still can break [ˈkœnn̩] into morphemes: [ˈkœn.n̩], which cannot be done with the other variants you suggested.