Whats the ideal workflow for converting 16 bit prophoto to 8 bit srgb ?

I'd look at a perceptual sRGB ICC profile  if the source image has a significantly wider gamut and details may be lost in the conversion in out of gamut areas. Standard sRGB profiles will clip OoG detail, they will use Relative Colorimetric rendering intent even if Perceptual intent is selected as they lack the Perceptual transformation table. This will potentially have more visual impact than 16 bpc vs. 8 bpc.

Like Digitaldog I'd do the resize manually and perhaps ad da bit of sharpening (unsharp mask) 

Starting with the Prophoto edited file

1: archive that edited original (with it's adjustment layers) as a master copy, full size, 16 bit, ProPhoto.

2: make a copy, convert to sRGB (probably best to do this whilst still in 16 bit)

3: resize to the right pixel dimensions for the relevant site. 

4: Unsharp mask as needed

5: Make it 8 bit

5: 'Save as' a Jpeg, I prefer'Save as' to 'save for web' or 'export'.  I like to see what's happening 

A note on Jpeg

Jpeg is not OK for archiving or for any file that may need to be resized or cropped down the line. 

Jpeg is the worst possible format if you want to keep high quality - you should always archive a copy of your original, with adjustment layers intact - if that’s how you work.

Jpeg compression (at any setting*) really is "lossy”, irreversible and cumulative, so should ONLY be used only for final delivery AFTER resizing & cropping to the FINAL size and crop.

Why? Any edits to size or crop, or even just re-saving a Jpeg file means further compression, potentially that’s very damaging.

The jpeg damage is not always immediately apparent, which is perhaps why it's still widely used - however, the compression will soon cause issues if you do further work and save again. That’s when you’ll see a jpeg with some real issues.

*don’t imagine that selecting maximum quality for your Jpeg is preserving the original data, it's still compressing a lot which discards information.

I hope this helps
neil barstow, colourmanagement net - adobe forum volunteer - co-author: 'getting colour right'
google me "neil barstow colourmanagement" for lots of free articles on colour management

Expect for Image crop and resize; there's really no difference.

Resize in high bit vs. 8-bit per color; sure, but I doubt that will make a difference. I'd let the conversion to 8-bit per color and then conversion to sRGB/JPEG be the last step.

Important consideration: all editing in a wide gamut color space (ProPhoto RGB) and a high bit (what Photoshop calls 16-bit). And yes, save a copy to keep the original high bit, wide gamut, high-resolution data. Spin off the sRGB/8bit per color, JPEG from that when you're ready to supply that for whatever need you have for such a document.