Microsoft may have very modest goals and they may not want to add to the bottom line directly through Surface sales as much as make a statement. This would be similar to Google and Amazon who have ulterior motives for selling their devices (to get people into their ecosystem). While MS is in transition and may end up becoming more like Apple eventually, they are not Apple and do not rely primarily on hardware sales (which then also feed the Apple ecosystem).
MS doesn't need to directly compete with iPads or Apple but what they do need to do is make sure Windows (their bread and butter) doesn't become irrelevant in a post PC world running on mobile devices. If they can establish Windows as a player in both PC and mobile worlds then they are going to continue to be a dominating OS force. So I think it is relative to their over all goals. If they made 400,000 units and sold 400,000 units in two months then the sales are strong. Maybe they will eventually expand beyond their current constraints but this was not a mistake.
MS took a specific business approach and their choice was not to mass market the Surface devices. There are a variety of reasons that could have led them to this decision but just because they didn't put it on every retail shelf in America or around the world doesn't mean they are failing. Perhaps they could have sold more and will sell more if they do that but there is also nothing wrong with limiting their sales either. Assuming that the end game is to sell as many Surfaces as quickly as possible is a mistake many people are making in comparing MS and the Surface to other tablet retailers. For that matter Google wasn't the first to ship Android tablets and still has limitations on where you can buy its nexus devices.
You make some good points, but I feel that MS has made a huge marketing mistake with both the Surface and with Windows 8 in general. Perhaps their thinking is exactly as you describe, perhaps not. I don't know. As a long time Linux user I was never a huge Windows fan but have been running 8 on one of my laptops for a while and have come to like and respect it as a very good operating system, and it led me to buy a Surface to compare against my iPads and Android tablets.
I feel that MS has a great product (not perfect but none are) and they could easily command a respectable and profitable market share in the tablet market if they seriously market the product. Every single person I have shown the Surface to has been impressed by it.
I personally never said they are failing with the Surface, but I do think they are making a huge mistake in not marketing it better, which could lead to failure in the near future. It makes little sense to put a product on the market you don't intend to sell. No sane company would do that. MS intended and intends to sell the Surface.
Yes, it's possible the RT was put out just to test the waters (a timid move IMHO) but they do want to get into that product line or they wouldn't have developed the 8 OS or the Surface in the first place. I think MS is moving in the right direction with both the Win 8 concept and the Surface concept, but their lack of aggressive marketing might keep them in third (or fourth) place in the tablet OS market.