OK, understand, and I've heard this before. I actually don't have Multisim, ADI has changed to ADIsimPE. However we do have some SPICE experts to support such questions as yours. BTW there is an LVDS driver shown in the AD8333 data sheet in the eval board section. I'm pretty sure this was tested and works in SPICE but the topic hasn't been raised in several years. From what I can tell from your attached schematic the LNA + ac coupling may be OK but we have always used dc coupling to the AD8333, and the input circuit shown in the schematic on pg 29 of the attached. Also based in the scope response in your schematic, the output looks a lot like a sine wave with some LO in the output, especially if you are using it at a lower frequency without changing the LP filter in the AD8021 feedback. What is your input frequency? If you're using the circuit in the data sheet bear in mind it was designed for an input frequency of 5MHz. For any other frequency you'll have to scale the filter appropriately.
Now that I think of it, there is a pretty good (maybe even better) design guide in the AD8339 data sheet.
I hope some of this helps,