Electrical – Male-male flat HDMI cable

High speed cables use shielding, twisted pairs and separation etc to mitigate intereference. Each standard has a maximum length that helps ensure that it can deal with exepected background noise over that distance. So, assuming that you are not using long cable runs, the connector you choose is a low loss type and you try and maintain as much of the original shielding/twists etc( eg do not expose 30cm of cable at your join) it will probably be ok. But until you make the connector you are probably not going to know.

You can always make the traces 10mil right after they leave the esd diodes, that's what I do, route 10mil as close as you can to the esd diodes, end that trace, now continue where you left off but now hit shift-w I think, and select 6mil trace width and carry on the last 1/8" or so to the esd diodes. 120ohms isn't that much of an impedance mismatch if you've only got a short length, you think the 1/16" bond wires in the chip (U9) are 100 ohms differential? transmission line effects can often be ignored when the discontinuities are less than 1/10 of a wavelength (include up to the 5th harmonic to be safe, so bandwidth in gbits/2*5 = 5th harmonic frequency (in ghz), speed of light/frequency = wavelength, if your discontinuity is less than 1/10 of a wavelength (1/20 is even better) then you probably don't have much to worry about. 120ohm lines wouldn't be a major issue unless the entire HDMI cable was 120 ohms, even then it'd probably still work as a lot of HDMI receivers now have cable equalization anyway which partially compensates for cabling oddities.