It is unusual for the bottom end to go at 100k miles in a 3.0L Mitsubishi engine, but at the rate it's been using oil, there's quite a bit of cylinder wear. I suppose it's possible that the mileage wasn't accurate when you bought it, but not likely if you bought it from someone reputable.

I assume the mechanic looked the engine over carefully. If not, the knocking may be a loose harmonic balancer (crankshaft pulley) - a known issue from improper bolt torque and a bad bolt design - now superceded by a new bolt that is available from dealers for a few dollars. If this isn't the problem (and it probably isn't), the mechanic is probably correct. Further driving in that condition will quickly destroy the engine. The oil consumption rate indicates that it's due for a complete tear-down and rebuild, anyway..... unless the oil is leaking to the outside. Do you see any oil stains under the engine after it's been parked overnight? If so, it's possible that a seal is leaking and you could get by with replacing the seal and some bearings - still not cheap to do.

Judging by the limited mileage, I'd suspect a piston wrist pin issue from lack of use, but it's hard to say without a close look. Either way, a piston/rod/bearing issue is going to be a major expense, so be prepared for some bad news..... up to and including a full rebuild.

Frank