For the problem of Displaying a Thumbnail of the video recording...there is an event called PreviewFrameAvailable on the AudioVideoCaptureDevice. I think if you setup this event handler it will tell you when the data is available to get a preview (image/thumbnail) of the video.
There are also 3 methods for grabbing a byte array of a preview image (GetPreviewBufferARGB, GetPreviewBufferY, GetPreviewBufferYCbCr). All three of these methods return a byte[] of the pixel data for the image in the corresponding format (ex: GetPreviewBufferARGB should return the raw bytes of a raw bitmap in ARGB format). After getting the byte array of the preview data you should be able to encode it as a PNG or JPEG or whatever compressed image format you prefer. Best of luck.