Dear Elena, 

Sorry for the late response. and I'm happy you get answers to some of your questions

For SDM, a empty column for the t_thr is the same as fillinf it with NA, so it's the same.

Indeed, if you get an empty webpage is that there is no significant results for the contrast/threshold you set.

You can use the randomise tstat as input for the t-images in SDM. 

The only way to make FWE faster is adding more CPUs in the prompt, but there is nothing else to do

Best, 
Lydia