The garage could, and acknowledge that there's a chance that'd work out fine. Their concern is that low oil pressure has contributed to the spring failure, which is difficult to prove now without tearing the bottom end apart as well. Although they admit that while the cam lobes have damage from the spring, the cam journals aren't worn.

They felt that it would be a gamble buying a second hand head, timing kit, gaskets etc and finding oil pressure is low. Finding a working complete engine and fitting that is marginally more expensive, but if the engine they source is a dud they'll take the labour on the chin to refit another from the breakers. They are not happy guaranteeing the current block will be fine. I see where they're coming from.

You are clearly a knowledgeable competent mechanic and I cannot imagine how your oil change contributed to this disaster. There's a lot on Honda sites about this spring problem causing just what you have experienced. If the pistons have been slamming into the valves, albeit briefly, I would try and source a replacement engine from a write-off rather than rebuilding it, and make sure it gets an Italian tune-up occasionally!

I believe oil spec is either 0w-30 or 5w-30, they were filled with 0w30 at factory unless I’m mistaken.

Last 1.8 I had, Honda told me and I`ve heard it from a mate that works for Honda, they had some cars that burnt a small amount of oil, so 5w 30 was put in instead of the 0w 30 which cured it

but they are designed for high revs which some people will not do, they are designed to take to red line/limiter without any problems, and if you check on the net some are turbo`d and rechipped without any more mods done and no one has any problems with them, except those that do not check the oil, though they have a warning system which appears to work well

I did read on Civinfo that a few did have spring problems but was mainly in US this is certainly the first I`ve heard of it here.