Yes it sounds like USB mode 3 option is not good for your current setup. Help me understand how your setup works since I never had the SmartHub in my Accurite rig. Where do you make the selection for which sensor reports to WU or other services via Meteobridge? It must be in the SmartHub since I don't see that option in the Meteobridge software. Am I understanding this correctly? Or do you get any additional selections, like you do with Davis, when you select the Accurite Internet Bridge on the weather station page of the Meteobridge?
It sounds like you probably already know this info but I'll tell you what I did when moving to the Ambient 2902 array just in case you do not. I never used the Accurite display to view my weather data and didn't want one from Ambient either so I was thrilled to see them selling the WS-1550-IP for $91.00, which is the 2902 array with the observerIP. The only additional item needed was a WH32B for the barometer, also adds indoor temp and humidity, for $15.
If you can't get your Accurite setup to work the way you want, I think this option is a pretty good compromise at an affordably price. Since the temperature accuracy of the 2902 array is reported to be more accurate across the board when compared to the 5in1 you may be able to do without your separate sensor, the only item left to deal with is the rain sensor. Its only a guess at this point, maybe others can chime in that have had the 2902 longer, but the design of the 2902 looks like it will be better in the spider department than the 5in1 is.
Its a shame that I've already given away my Accurite setup to a friend whose station died or I would give it to you to play with. It would be interesting to test if the temp, humidity and rain gauge could be disabled on the 5in1 to see if the display panel would pickup the readings from your shielded tower, 899 rain gauge and wind speed and direction from the 5in1. If it worked then you can use USB mode 3 to get the info into your Meteobridge. Wouldn't that be nice!
Good luck