I've worked on more of those Basildon engines than I care to remember, and never have I heard of an issue with a cracked oil gallery putting oil in the water. To be honest, even if it was cracked, I don't think it's close enough to a water passage to do that.
Furthermore, I don't see how the oil gallery could be replaced with a tube because the lifter bores intersect the oil gallery. It would be darn near impossible to do. Are you sure you understood this repair correctly?
Back to your original problem: Pretty rare on that engine, since it does not use an oil cooler, unless the BSD engine came with one. Is there an oil cooler behind the oil filter? If so, have it checked very carefully, or take it off, it is not really needed.
How much oil in the cooling system are we talking? It doesn't take much to make it look like a lot. Is there no evidence of water in the oil? It's possible that a pinhole low enough in the cylinder wall could allow oil to be scraped into the cooling system I think.
Other than those 2 things (oil cooler and pinhole), I can't think of how a 201 could do that.