Oxepin is certainly not aromatic. If it were flat and the oxygen atom was sp² hybridized, then it would be an 8π-electron anti-aromatic system, but to avoid being anti-aromatic, it exists in a boat-like conformation, with incomplete conjugation of the π-system.^Ref Interestingly, in solution, oxepin is in equilibrium with benzene oxide - the two interconvert via electrocyclic reactions.

In terms of the broader questions about Hückel's rule - it's important to look at whether the system is cyclic, planar and has (or could have) good overlap of p-orbitals on each atom within the ring. The 4n+2 π electron part of the rule is only relevant if these prior requirements are met.

In terms of heteroatoms, they can "choose" to be hybridized in such a way that a lone pair in a p-orbital overlaps with the rest of the π-system and leads to aromaticity, eg. furan, or to be hybridized in such a way as to avoid or minimize overlap, eg. oxepin. If geometrical constraints mean that a heteroatom's lone pairs can't overlap effectively with the rest of the π-system, then that breaks the conjugation and the 4n+2 π-electron rule is not relevant.

Ref: E. Vogel, H. Günther. "Benzene Oxide-Oxepin Valence Tautomerism". Angew. Chem. Int. Ed., 1967, 6, 385–401.

Oxepin is certainly not aromatic. If it were flat and the oxygen atom was sp² hybridized, then it would be an 8π-electron anti-aromatic system, but to avoid being anti-aromatic, it exists in a boat-like conformation, with incomplete conjugation of the π-system.^[1] Interestingly, in solution, oxepin is in equilibrium with benzene oxide - the two interconvert via electrocyclic reactions.

In terms of the broader questions about Hückel's rule - it's important to look at whether the system is cyclic, planar and has (or could have) good overlap of p-orbitals on each atom within the ring. The 4n+2 π electron part of the rule is only relevant if these prior requirements are met.

In terms of heteroatoms, they can "choose" to be hybridized in such a way that a lone pair in a p-orbital overlaps with the rest of the π-system and leads to aromaticity, eg. furan, or to be hybridized in such a way as to avoid or minimize overlap, eg. oxepin. If geometrical constraints mean that a heteroatom's lone pairs can't overlap effectively with the rest of the π-system, then that breaks the conjugation and the 4n+2 π-electron rule is not relevant.

1. Vogel, E.; Günther, H. Benzene Oxide-Oxepin Valence Tautomerism. Angew. Chem. Int. Ed. 1967, 6 (5), 385–401. DOI: 10.1002/anie.196703851.