The Collision theory occurred during the late stages of accretion so there was lots of junk swinging around the solar system. Theia probably co-accreted in the same orbit as the Earth in either the L5 or L4 point, then was perturbed into a walking orbit that collided eventually with Earth at a very low hyperbolic excess.

If Theia could co-accrete with the Earth then there's no reason why another body couldn't co-accrete with Venus, do a swing-by, then smash into Mercury to strip its mantle. Also possible is the accretion of Earth & Venus relatively close to each other, then their subsequent migration and disruption of the forming of Mars and Mercury, leaving them lower in mass.

All physically reasonable possibilities. What's needed is observational tests, probably geochemical, to distinguish the possibilities - a scientific "smoking gun" to cull the possibilities down to what really did happen. For about 20 years the Collision theory has claimed to be the only player in town, thus winning by default. Now Malcuit's theory is gaining ground. Only observations on Mercury and Venus will tell them apart, though just what I'm unsure as yet. Maybe MESSENGER will dredge up more evidence either way, or maybe the proposed Venus Flagship mission for c.2025.