I watched a Shimano Video about cleaning these reels and they said that the bearing in the antireverse mechanism should be cleaned and dry and not oiled or it will slip and ruin it. Why can't you just replace that antireverse bearing and see if that fixes the reels antireverse function.
The video said to not overoil the bearings. Just clean them with a Q tip and a dab of alcohol on the end of the Q tip. One drop only.
To oil the bearings for the first time (one time only deal) put one drop of oil on a Q tip and then lightly touch the Q tip to the bearings one time and wipe around the bearing. You only want a tiny film of oil to go on the bearings. After this initial oiling they suggest a routine cleaning with Q tips and a cloth after each days fishing and one a year more thorough cleaning and regreasing the drag washer and gears with teflon grease from Shimano. Don't use too much grease either. Just a little smear of grease on the finger tip and then rub the fingers together and only then apply some grease off the fingers into the Carbon Fiber drag washers.
Clean the races with Q tips and clean the pinion gear with Q tip and Alcohol. There are certain washers that are not flat and are concave and you have to put them back together so that they have a space between them. So pay attention on how things are installed on the reel before you take them off for cleaning and put them back on the same way. There are two springs under the side cover and you have to be careful not to lose them. And there is a plastic part that only fits back on one way.
What model year was the reel made. Perhaps you can sell it for parts. I'm surprise that Shimao could not fix it for you though. Where you trying to get it fixed under warranty? Did they offer to repair it after the warranty was out for a price?