Toyota Repair: Toyota Sienna Repair, valve stem seals, piston rings

Question
I took my 1999 sienna LE  in for a diagnostic yesterday at Made in Japan auto shop, They foud Multiple codes.
These codes suggerst Random Missfire, Missfire in all 6 cylinders and Cat problems.
P0301
P0302.P0303 P0304 P0305 P0306, & P0420
They want another $100. to scope test the coils and another $100 to remove, inspect & replace the cat converter.
My Sienna is running very poor, as if it's not running on all 6 cylinders, Burning about 1.5 quarts of oil per week and smoking very bad.It's been running this way for about 4 months. I took it to the dealer 6 months ago and they said I only had had a bad cat converter. At that time the car seemed to be running fine except for the smoke and burnning the oil.(no missfires). Is it possible the cat went out first and ruined the coils? or did the dealer just not see that it was missfiring and the missfires are causing the cat codes? What can I do myself from here to fix the problem without going back to the shop? Thank YOU.

Answer
The oil burning issue is probably the root cause, excessive oil deposits on the spark plugs will cause misfires, the catalityc converter will fail because of the misfires, it will probably have to be replaced but the cause for the misfires must be determined first, I would recommend having a toyota dealer diagnose the problem again for the excessive smoke, this problem needs to be addressed first, have the engine compression tested, have one of the valve covers removed for inspection of excessive sludge buildup, if there is, the piston rings are probably the cause if not a replacement of the valve stem seals may take care of the problem, in any case this engine has some serious issues, replacing the converter or ignition coils won't fix the problem.