"That's for making new bows, what about enhancing existing ones? I assume it plays no part?"
That assumption would be correct.
"I just learned today that enhancing stuff adds a fixed number of props at fixed intensities, no randomness to it- is that true?"
Partly. All woods except for Heartwood have fixed properties that will be added. Here is a link to the different properties per wood type:
Wood Properties
Heartwood gives a random mod from the list on that page, everything else is fixed. You'll find that most enhancing is done with either Ash or Yew. Oak, while a low level wood, tends to fail quite often as it is a luck enhancement, and any weapon with multiple mods adds to the difficulty of the enhancing chances.
Just always keep in mind 2 things:
1. You never enhance something you don't have a replacement for, so if there isn't something that you can use with similar mods, you really need to think hard about that enhancing failing, and the bow breaking.
2. You never really own a bow until it survives enhancing, unless you decide not to enhance it.
I tend to stack up a bunch, then just enhance away, not even looking at what it is I'm enhancing until after I'm done. Less heartache that way.
