When it's oil change time, just take it to a dealer and ask that when they do the multi-point inspection if they could also check for any leaks or cylinder head cracks. It's still under the powertrain warranty and anything found will be covered
Ask the seller if you can take it to get inspected. Then take it to a Mazda dealer for a used car inspection. Depending on labor rates in your area, should run about $150
He's already using 5w-30 with the turbo.
It's a 99% chance of it being the stem seal issue that there is a TSB for. He just needs to get it to the dealer
You really should rotate the tires at each oil change
When you call them just ask if they do a multi-point inspection with oil changes (almost all places do). Then just say that you want to make an appt for an oil change and tire rotation
For control arms, nothing else on the suspension has to be touched other than the control arms. There is no "taking apart half the suspension" involved
It's much easier to replace the control arms and probably less labor too. A 9 year old CX5 with 100k+ miles is a perfect candidate for control arms, esp as such a reasonable price