I've run into the following situation a few times and am hoping someone might have a solution:
The possible solutions I can think of are:
None of these are terribly appealing to me -- has anyone encountered this and discovered a better way?
You should control order of operations with a wait step on the data changes in Marketo in this case. Without seeing the screenshots, I can't offer more than that.
Blocking field updates will indeed block all updates, not just the ones that happen after conversion.
I would rather use the first approach, also recommended by Josh above, or the third one. You can trigger the smart campaign only on the "lead is converted" trigger.