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That's ROOLY good despite the optical issues mentioned. You've got a lovely balance between showing bright detail and faint features, preserving the overall feeling and structure (rather than trying to show every last photon in the dark regions and ending up with a dull flat image as some do).
Short OIII and long Ha flats will happen if you use a blue light source (like early evening sky or an LED with not much red in it). The reverse happens if you use an incandescent bulb for flats.
Defocusing the guide camera should not normally cause guiding issues unless it's to the point where the guide star is too faint and blurry to see. Slight guider defocusing can actually be beneficial for those where the guide camera pixels are big or the guider focal length is short, as it spreads the guide star over more than one pixel and gives a better estimate of the centroid. Severe defocusing to the point where the software can't see the guide star at all will of course be catastrophic, but that would produce a squiggly trail, not what you had.
Bad seeing always affects OIII much more than Ha. Also anything like a flappy heating wire for a dew shield or secondary mirror heater will produce worse diffraction spikes with OIII.
Despite all that, one of the best, most pleasing Lobsters we've seen.
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