Is the engineer likely to look at my graphs if he sees that they come from a "hacked" HG612?
From experience, some engineers will be keen to look some won't even realise you can hack the HG612.
The engineer I last had was great, he saw the MDWS graphs and said something looked wrong and off he went, however in the past I've had an OR engineer who didn't understand the graphs at all 
All depends on the engineer you get
That's exactly what I found when I had numerous engineer visits until my intermittent HR fault was finally found & permanently repaired.
Since I had FTTC installed back in 2011, I have noticed that attenuation gradually increases (with lower sync speeds) during summertime & decreases (with higher sync speeds) in winter.
It's currently on the increase (see attached).
It is usually at its highest in August & at its lowest in January/February.
Your fluctuating SNRM, Sync & Attainable rates, Errored Seconds & other error counts over a long period would be the ones to show the engineer (if he/she is interested).
I'm not sure that your intermittent logging will have captured all the relevant details, but there should be sufficient to get a 'feel' for how your connection performs.
I did notice previously that you had one or 2 large/spurious spikes in your graphs due to intermittent logging that masked the underlying issue to an extent.
If you wished to upload a copy of your
modem_stats.log again I could manually edit those spikes out for you.
