Nice one Ekka, first some caveats;
--its virtually impossible to make accurate appraisals from photos, we get asked to do lots of times, to quote from a photo to make diagnosis etc...
--the colours and definition is never enough to be certain
But as always you raise some great points that need to be thought about and addressed.
Would I have said it was OK?....We would all get asked this question many times every week, is my tree safe, can you make it safe(er) will it stand up to a blow?
Nobody can ever say a tree is safe, every tree even the most healthy vital specimen will fail in the right (or wrong) circumstances, they are a natural system, and natural systems fail. If you start assuring cliants that a tree is safe you will get yourself and them into trouble sooner or later. All trees have some defects in them that too is part of being a natural system, how important those defects are for the client/owner relates to target..no target no problem. Your tree clearly had a target the neighbours yard and presumably potentially the people who use that yard. (You can calc real figures for target occupancy, value, impact potential and risk of harm but maybe thats for another thread.)
There are some things that I think I see in the photos so I'll point those out.
On the stem of the fallen tree there is a area of altered bark Circled in red, not sure if this was present before the fall, but it certainly looks like a large injury?
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The appearance of the fibres that have failed, their colour and the type of fracture strongly suggest fungal decay to me. The variety of brown rots we get in our Eucs are often very hard to pick, you don't always see mycelium in the wood tissue, and you don? always get fruiting bodies particularly if the environment has been dry.
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There's a second odd looking patch on the stem just above ground level, which also suggests decay.
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The lack of a root crown formation at the point at which (what should be) the buttress roots appear, and the appearance of a continuation of the stem diameter above the roots, below them leads me to suggest despite the appearance of being properly planted the tree has been buried by fill.
Root crown proper and the real buttress roots have died and decayed and the adventitious roots have maintained a canopy but not been able to support the tree under wind loading.
Adventitious roots.JPG
This is all supposition but I have seen this very same event numerous times at Currumbin Hill over a year ago. Fill buried trees after 10 - 15yrs can be very hard to pick, the canopy remains vital and healthy looking
SF