Question

where are electons most properly located in a molecular bonding orbital

Answer 1

In circular orbits around each nucleus.

Answer 2

the outermost shell holds always the electrons involved in covalent bonding

Answer 3

Phascolarctos is correct, as ever.

The outermost electron shells of covalently bonded atoms are the ones involved in sharing electrons. They accomplish this by blending together - that is, they stop being atomic orbitals around their individual nuclei and become molecular orbitals which extend around multiple nuclei in the molecule. Without getting into the quantum chemistry of it, molecular orbitals have energy levels just like atomic orbitals and unless otherwise excited, bonding electrons should be in the highest occupied molecular orbital or HOMO.