Thank you for your thoughts! Long post in response.
The smokey smudge stain, for want of a better way of describing it, isn't under the glass, unfortunately, I had tried that, it's within the panel itself. It's a not uncommon problem with the 2010 model and Apple, after initially blaming smokers, which I am not, blamed the panel manufacturer themselves.
I frequently vacuum all the vents and when I last had the iMac open to fit an SSD and replace the bass-rumbly Seagate hard drive with a larger capacity Seagate model (I believe Apple solved the bass-rumbly problem by changing the desktop hard drive for laptop models; I used soft silicone grommets, which helped but using the SSD as my boot drive significantly reduced the problem, I could use Disk Utility and unmount the drive, for that matter), there was no visible dust on or around any of the fans.
I fitted the SSD where OCZ and others advise, beneath the optical drive. Temperatures before and after weren't significantly different, so I went with it. A possible plan is to remove the 2TB replacement hard drive and put an SSD there, along with the current SSD. Or else I could do what Apple apparently did with the eventual and long overdue recall of 2010 iMacs and replace the hard drive with a laptop hard drive; I resisted doing that because I am used to service agents putting dings and scratches in the facia of whatever they are time-pressured to repair. Plus, I trained as an AV tech and, apart from being careful with the ribbon cables, one of which was glued down which wasn't mentioned in the ifixit disassembly tutorial (my one anxious 'why isn't this ribbon cable coming out?' moment) everything was surprisingly easy.
I thought I would perhaps take power from the un-needed optical drive for a slot fan; I wouldn't mind if it was always on, as long as it's quiet, or I could fit a tiny switch along the bottom of the iMac. I had thought of having an eSata out but have no use for eSata. Other things I have thought of doing with this model. As far as I know, the pcie socket containing the airport card, can't be used for a USB 3.0 card but I'm a little out of date with that one. I seemed to be one of the very few people asking and didn't want to be one of the first to try. Ultimately, when the next generation of NUC models come out with Skylake M chips with Iris Pro graphics in late 2016/or possibly early 2017, I would like to gut the iMac and replace the internals with that, if those NUC models aren't horrendously expensive. It should be much cooler running. On the other hand, I would have to find a way to have target display mode work, so I can use the monitor with my Hackintosh. That is the main use.
I think I am stuck with the screen stain. People who have tried to carefully prize the two halves of the panel apart and clean it have sometimes found they could remove some dust, others have been frustrated to find that the stain was permanent, while others found when they reassembled, the panel no longer worked. Those who have tried advised me not to. The stain is annoying while on a white or grey background, particularly if I have a finder window open in that area for a long time; the stain retains a faint white ghost of that Finder window, specifically the folder icons and text, which vanishes if I bleach it out with a fully white screen for a while. I can just about live with it, though it does make re-sale impossible, unless I source a replacement panel, which I would do if it was an opportunity to upgrade, though that would present additional problems. It doesn't bother me when gaming. What does bother me is how loud the iMac is as a screen when gaming from the Hackintosh. Widening the optical drives slot won't be fun. I might just carefully drill out some holes, there, and try and make them look as meant as possible. I don't usually look at the iMac, side on. Whereas people who enlarge the slot at the top back, even done neatly, it stands out like a sore thumb that Ives would have had nothing to do with.
Thanks for your thoughts. It's frustrating not being able to find other people who have done this. I thought it would be the most elegant solution, especially the number of people I found who complained that heat was damaging the screen (vertical lines) and likely reducing the life expectancy of the components. My temps have always been at the upper end of what was deemed acceptable by Apple. I used a fan control app but that defeated the purpose of having an iMac, the relative silence. Plus, it seems unachievable when gaming via the external Mackintosh, when gaming under Windows 10.
