Spraying out of the pressure cap? I guess you mean the foam was coming out? Sounds like you mixed incompatible coolants, hence the foaming, i would suspect this is the problem and not the gasket. The oil should be changed every 12k miles at least, when did you last chage it?

The focus should be fine if you change the oil, coolant and pressure cap. This is all quite cheap to do.

Almost certainly head gasket.

You dont tend to get the mayo under the oil cap like with a petrol engine.

I'd say its worth getting it repaired as its mostly labour, Focii are usually very reliable actually. They should be just as good as the Mazda if treated the same.

I'm no expert but mixing antifreeze brands should not matter, but mixing different types of antifreeze may well matter. Car manufacturers seem pretty insistent about the type of antifreeze in their engines and this is because they know all about the different metals and plastics and rubbers used in their engines and rads and ahat types are compatible with them. It is best to stick with the recc'd type, e.g glycol based or organic acid based. Using the wrong type may result in long term corrosion or seal failure due to chemical reaction. What happens in the short term depends on exactly what gets mixed together. Some time ago I faced the dilemma of using the "wrong" type of antifreeze or waiting till I could get the right type, so I experimented by mixing samples in a glass jar. The result was a glutinous stringey deposit and I certainly did not want this inside my engine, blocking narrow water ways. If you have any unused samples an experiment may be enlightening. It is possible that the mixture in your engine is no longer a homogenous free-flowing mixture, If the foam is being generated deep down rather than just at the surface where it is in contact with air then it's coooling ability may be reduced. I would be inclined to flush the whole system and refill with the recc'd stuff.

Regards