9sp/10sp compatibility in Shimano SLX drivetrains

Question

This is related to my previous question about SLX chainrings.

I'm half way through upgrading my drivetrain from 2x9 to 2x10. I currently have:

Shifters: SL-M670-I (10sp),
Chainset: FC-M665 (9sp double with bashguard),
Rear mech: RD-M675-GS (10sp),
Front mech: an old Deore 3x9 that doesn't play well with the other parts and needs replacing.

I'm trying to make this drivetrain work at as little cost as possible, and I have two concerns:

I understand that, officially, the M665 chainset isn't 10sp-compatible, but people have reported (in this thread and on Bikeradar) that it works fine. Are there any complications in this?
I shouldn't use the newer FD-M675 front mech because the chainline doesn't match that of the chainset I have (48.8mm and 46.8mm respectively). That leaves me with the FD-M665 front mech, but will this be compatible with my shifters?

Edit: This thread suggests that the 9sp front mech will work with the newer 10sp shifters. Can anyone confirm or provide more information?

In response to PeteH's comment: In short, what I don't understand is:

Why exactly should the M665 chainset not be 10sp-compatible, while the M660 is? Are the chainrings too wide, and if so, why do people report that it works anyway?
Do the 665 and 675 front shifters have different cable pulls? If not, is there any other reason I've missed why the front mech from one groupset might not be compatible with the shifter from the other?

Unfortunately, the cable pull ratios are elusive and seem to be unpublished.

That you say you're halfway through this upgrade, and you're asking this level of question, seems a little odd. Do you understand why manufacturers claim that 9sp groups are not compatible with 10sp groups? About different chain widths? And why people say they are able to workaround the differences? These will all help toward understanding what you're attempting here. — PeteH, Jan 7, 2015 at 16:58
@PeteH Yes, but in my experience (admittedly mostly with road group sets) the incompatibilities tend to be at the back of the drivetrain rather than the front. I've never had any problems mixing various flavours of 9sp and 10sp Shimano components at the front of the drivetrain on my road bikes, so long as the rear end is set up properly (which it is, in this case). But then, maybe I've just been lucky :) — Will Vousden, Jan 7, 2015 at 18:22
If it helps, I ran "10-speed" FC-M610 in 3, 2 and 1 ring configurations and didn't experience any compatibility issue with the rest of 9-speed drivetrain. — Klaster_1 Нет войне, Jan 9, 2015 at 8:51

Slovakov · Accepted Answer · 2015-01-09 12:01:10Z

The difference in chain width between 9s and 10s is less than 1mm so it won't make much of a difference once the mech is adjusted correctly. I have M660 on my bike and M665 on brother's bike and they aren't so much different, so if 10s is proven to work on one, it should work on the other.
Front shifters have the same cable pull, but front derailleurs have different travel rate between 9s and 10s. Therefore if you want to use a 10s shifter with a 9s crank it's actually better to use a 9s derailleur. Although both would work, a 9s would be more accurate in this case.

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