The defrost looks to be working by that frost pattern, the issue seems to be with the draining of the trough as suggested - be careful with your heatgun in that one as the evaporator floor is comprised mostly of styrofoam and can be deformed easily - try running a small wire/zip tie etc through the drain (loops like a trap) - use a steamer or compressed air to clean it out. I have seen cases where the styrofoam becomes "logged" from exposure to freeze/thaw that it breaks down the ability of the foam to shed water and instead kinda soaks it up. I had the joy of replacing the styrofoam divider (NOT a fun job) that runs between the compartments.
If you have the tape that CTG recommended you can apply some to the area around the drain and see that the slope goes inward to the drain, raising the front legs of the fridge may aid in this as well.
Key thing is to make sure the drain is unrestricted from end to end and will actually carry the water to the drain pan that resides near the compressor.
Looking at the parts listings, it appears that they have some sort of ridiculously priced "evaporator drain pan kit" which may be to address that issue, obviously you would replace the fridge before buying this kit. Sadly, R/C has no picture of the kit so I dunno what's all involved - http://www.repaircli...ber=PDS18SBMRBS
I do not have an inverter M/W, but several people I know have them and they report very happy with them - I have only ever looked at a few of them technically, but only a serious issue with one. Others I know who service them tell me that often the Magnetron and Inverter board need changing. The one I looked at required that but the owners purchased another one that was the same. Just over a year now and they quite pleased with the new one. The Inverter units are definitely lighter and have more functional oven cavity space as the Inverter takes up much less space than the traditional Transformer/Capacitor etc. Microwave pricing has dropped so much that most major repairs become uneconomical akin to Refrigerator System repairs.