Did the sensors go to 0v after it cut out? Have you checked all the basics like plug gap etc. I know it sounds common sense but seems like there is quite a bit of guess work going on and no figures or readings to be used for diagnosis.
Sorry but with no engine codes this would be hard to diagnose unseen, my honest advice is take it to a main dealer, consult 3 has the option of leaving the box plugged into the car with set triggers so when the fault happens the data can be retrieved at a later date for further dignities.
Remove the rocker cover and measure petrusion of the hydraulic tensioner, 10-12 mm or over and if say chain has stretched. The mpg could be to fuel trim as its rough running.
Buy a cheap lambda sensor socket off eBay, save having to remove the heat shield and poss snapping rusty bolts for the front . For the rear just jack up o/s of the car and you can get a spanner in. The rear connector can be felt behind the engine and unplugged ( it's a green connector on a...
Ask the mechanic to recheck using live data from ecu, the front sensor can appear to stick at around 0.3v which is evidence of a bad sensor but lean over the motor and unplug the rear one. If the front voltage jumps up it will be a fault with the rear holding the front low.
Check the earths, there should be one above box going to bulk head, one on each inner front wing behind head light, rear d post or quarter panel and one under dash top behind radio
The drum can be heated and hit out evenly with a big socket sitting on the edge of the bearing race. Thinly coat the new bearing with copper slip and repeat procedure to refit. Just remember the circlip. K12 esm states 140-210 Nm
Most new drums come without the abs ring that pushes on around where the bearing is on the inside. Did you swap the abs ring over ( sorry for stating the obvious but it's not a toothed ring like on a passive abs, its a smooth ring as its active abs sensors)
Have you checked the obvious, fuel, air and spark etc. any signal from cam and crank sensor? Replace the ecu but get the car details, you may have to get a new key cut to the existing ign barrel and code to new ecu.