Cobber

ECU calibration issue?????? unless your ECU has been remapped and badly at that.....then I suspect they don't have a clue what they're talking about perhaps they work to the principle of:
If you have no idea of what you're doing, talk a load of shit, with plenty of technobabble, gravitas, authority and a straight face, in the hope you bamboozle the audience.
Politicians TV presenters and senior management do it all the time!

I've always found the only way to remove disc and drum brake retaining screws it to use an impact screwdriver

give it a good whack with a 4lb (2 kg) hammer

and they should then undo easy

Actually I'd definatly give the car a service and do the timing belt, but other than checking for problems I'd leave the head gasket alone......Don't mess with success!
Compression tests, cylinder leak down tests, coolant sniff tests and coolant system pressure tests will indicate head gasket problems as will an inspection of the oil quality, snotty oil will not do! and is an indicator of head gasket problems. But it these all pass then leave the head gasket alone.
Don't rely on service records as the work may not have been done properly.

Some of you seem to have difficulty getting pictures to show properly in your posts,
I've found that the best and most reliable way is to ignore the image link icon button, it's above the text box

and use the attachment button, its below the text box.

Then press the Add files buttton

And then either press the Insert all button if you want all the attachment to display at once or press blue insert button below it to only display that individual choice of picture.
Once a picture is included you will see a green in message button DON'T press this

Finally you press the blue Submit button at the bottom.

Some states here in Oz have an annual mandatory inspection and some don't, there is bugger all difference in accident rates between those that do and those that don't, and those accidents caused by mechanical failure are relatively rare by far the biggest cause can be summed up as driver stupidity albeit inattention, farting about with mobile phones, good old fashioned lack of skills, not driving to the conditions, tailgating, drink/drug driving and the old favourite being a total dick behind the wheel.

An old mate who should've known better (he's a bloody mechanic) had a BMW 2002Tii, he wore the brake pads so far down they managed to effectively work like a lathe with a parting tool, the discs snapped off their hubs! It must've been making a hideous noise for some considerable time to part the discs from the hubs on both sides. The grating noise would've been bad enough, butit must've been virtually undriveable when the 1st disc failed, to continue driving it until the other failed beggers belife.
Mind you he is a marine engine mechanic, and boats don't have brakes....but still!

Clearly you need to do better..... rather than ricocheting off the bin it should've lobbed straight into it, where it belonged!
Must try harder next time.

That bolt is totally, absolutely, irrecoverably and monstrously fornicated beyond all redemption!

Get some of theive the offending oil on your finger and give it a good sniff, if its gear oil it should smell worse than a pole cat's arse!
Its up to you if you need to get a pole cat's arse onto your finger for comparison purposes!
I for one am happy to imagine that they don't smell good.
You should be able to smell gear oil, even if you're like me and have had your sense of smell largely destroyed by an industrial accident (chlorine gas spill)

The golden rule is ALWAYS suspect the new part!