I'd also suggest making Real Damn Sure first. Among other things, you can probably dump some UV oil
dye into the fresh water section. this is plain mineral oil
with a bright green (typically) UV dye used to check for leaks
in oil systems and AC leaks. IF if is getting sucked into the engine it will also show up (under blacklight) inside the cylinders, inside the engine oil, and on a clean handkerchief held against the exhaust
for a while. There are also water-based dyes of the same type. In any case--the UV dye will show up where the fresh water is going. Similarly, an engine oil analysis (about $25) should show excess water or other contaminants in the engine oil if the coolant is getting into the engine and past the rings.
Could be the mechanic
is right, and sure, and simply very conservative because he wants to have the engine out so the head AND block can both be machined perfectly flat before re-assembly. Could be. But maybe not.
Might be worth getting a second expert (ahem) mechanic down there to get a second opinion, and estimate.