I hadn't planned on making any sandbag models.  I'm not sure about the legality of using DoD:S sandbags, but that might be doable.  If not you could always ask Darkwulf to model some, I'm sure they would be useful elsewhere in EX.

The freestanding pillers, connected by a horizontal bar as seen here are known as "flying buttresses".  I now notice that you've removed them from more recent versions.

Yes, I expect all the nids to be able to get though the big doors, just the isle down the middle and between the pews was looking too narrow.

Regards your latest texturing I think I preferred the interior painted then metal, just my preference though.  Dark lighting with dramatic spot lighting could do the job of that dark texture just as well.

Coming along their Vault    Looking much better with the new textures.  Nice work on the stained glass.

As a guideline for tyranid sizes, the ex_dropzone entity was designed to be as large as as a carnifex with a brood of termagaunts.  Most Tyranid players wont be quite that big, but they do need more room to move than the guardsmen of the MDK.

If it's not too late the side windows might look more churchy of they were double height, rather than one window above the other.

Using a different stone texture along the bottom of walls and pilers gives a more realistic effect as seen here.

 This Shot shows another trick, adding thicker brick columns built into the walls also adds to the realism, the walls of your church seems a little bit thin to be needing flying buttresses.