Welcome to the SeeMeCNC Community

I got this printer to print what is basically a pinwheel, except longer.
Thin blades at very very shallow angles.
10" diameter, 1 full of spiral goes up 1", or slope of about 1/15 near edge.

And now I know shallow angles means it needs support.
So, lots of questions, answering 1 or more would be appreciated.

I'd like smooth surfaces, but i gather that wont happen.

So, I know slic3r will generate support, but does that happen in the middle of objects too or just from bed to bottom of part?

Is there a way to have those supports be in concentric circles? (so they don't affect the air flow much).

If I generate my own concentric supports, is slic3r smart enough to law out travel the "right" way so it crosses the supports?

How thin can vertical walls be?

How thin can slanty blades be and remain strong and airtight?

How far will it reliably bridge?

Should I run with fan on to bridge?

Thanks for reading.

for complicated parts, you will likely get better slicing results with Cura.

As for your questions...

smooth surfaces - .15mm layers with ABS will give you fairly smooth surfaces, and an acetone vapor bath after printing would make it very smooth

Slicer will generate support everywhere - including in the middle of objects - if it's selected. Cura gives you the option to only generate support material that touches the buildplate or everywhere.

No, Slicer is never smart enough to do what you want it to.

Vertical walls can be single walls. With a .4 nozzle, that means about .44-.45mm thick

How thin to remain strong and airtight? No way to tell. Properly calibrated machines can make airtight objects that are single walled.

Bridging is dependent on your machine calibration, material, and settings. I've bridged 80mm successfully with ABS. It sagged a bit, but it did successfully print.

Cooling fans shouldn't be used with ABS, and should be used with PLA, but again, results vary greatly depending on other variables.