Have you looked at replacing the cabinet door hinges to provide the clearance & stopping point. There are a pretty wide variety, one of which will probably fit your needs.

I think you're all right about the "prep" part of my small sink. It really is just a small second sink. I was thinking of finding a sink with a cutting board that fit into it so someone could cut fruit or veggies there. There's some with colanders that fit in, as well. I will keep looking for a shallower microwave; I'm sure I can find one. I've never used a microwave drawer before. Do they work well? I will have to do some searching in the appliance forum. It's not a bad idea to move the fridge...but unfortunately that space is short; I'm 5'2", and I can barely stand under it because it's a stair landing. I took the space from the understair storage so I could expand my lower cabinet/counter space. It has room for a shelf to hide an undercabinet light, but not an upper cabinet. I may do away with the tall pullout next to the oven and only do a lower pullout so I can add those extra inches to my counter space. I already have a nice pantry, so I don't necessarily need the storage. Aak! So many decisions! My thoughts for having the fridge in the middle was easy access from the whole kitchen, since I have a very large island. When I placed the small sink, I hadn't taken the space under the stairs for a counter, and I didn't think about moving the sink until it was too late. I guess I'll have to rebuild the house one day and get all the things I've missed right! Someone said you have to build the same house three times to get it all right. I thought he was crazy. After all, I've spent a year and a half drawing this thing and living in it in my head. Now, as we are in the finishing stages, I see what he means..oh well! :-) Side note: My cabinets are completely custom- made on site, and my cabinet maker will make each box whatever depth I ask him to. So, if I want the upper cabinet to be 15, 16, or 17" deep, he will be happy to oblige. That makes it better for the microwave, but worse for the sink.

With most OTR microwaves, I don’t think flush mounting is possible. Not because of the venting, but because of the the door swing. Typically, the hinge side of the door requires space beyond the 30” opening. On mine, the hinge projects almost 3/4" from the left side of the unit when opening, beyond the 30" width. Check that yours will open if it's recessed more deeply than usual between the cabinets. Below is a photo from above of the microwave door projection. The door thickness is just about the same depth as the top vent. If you recessed the top vent, the door wouldn't have space to open. Different models may have a better hinge setup.

Your salesman is wrong. The GE I have is listed at Lowes as available. I didn't check any other brand/model but I'd bet many, if not all, have the same capabilities. On my GE, the venting function (cooktop exhausting) is a separate fan and airflow path from the magnetron cooling airflow. Magnetron cooling always pulls air in from the grill above the control panel and blows from the grill at front above the door. The cooktop exhaust function can blow into an outdoor exhaust duct located at the top/rear (vertical) or back/rear (horizontal) if it's configured for outside venting. It exhausts from the top/front (as mine) when configured for indoor/recirculation venting. The dual squirrel-cage cooktop exhaust blower assembly is removed, rotated accordingly, and set back into the unit for which direction the output is to aim ... to the outlet port at top/back, at top/rear, or front grill above the door. There's also a plate that fits to cover one of the outdoor duct port directions according to which of the three exhaust methods is configured. This is a link to the installation instructions for the GE. There are three sections that describe how to orient the exhaust blower assembly and install the unit for which of the three exhaust direction/method is needed. GE JVM6175YKFS Installation