Let me shortly summarize the important physics topics for the sqrt{s}=500 GeV machine: a) measurements of the properties of the top quark in the continuum and at top threshold; b) measurements of the Higgs properties; c) measurements of possible new particles, for instance light supersymmetric particles and d) hints for new physics discovered via detection of deviations from Standard Model predictions, the so called ‘indirect searches’.
Starting with the easy cases:
Concerning item a) we should be still on the luminosity-safe side, so no problem;
concerning item d) high energy is favoured, so no lumi problems with the new undulator position is expected.
More complicated cases:
Concerning item b): the upper mass limit for a Standard Model Higgs is <207 GeV at 95% c.l., derived from the electroweak precision measurements and the LEP2 exclusion, so we would be just in the critical energy range. Most physics studies for the Higgs, however, will first be done at sqrt{s}=500 and at 350 GeV. Using the results of these studies at high energy would enable to optimize the needed energy steps for a threshold scan a priori. In other words, even for Higgs studies where we really need to go to the threshold, probably only a few energy steps are needed (Studies have been made showing that 3 steps are sufficient to reach the precision goal). So, suffering under luminosity reduction for maybe 3 energy steps is not nice but maybe bearable and from results at sqrt{s}=500 GeV and 350 GeV, we could already conclude whether we really need the bypass or not — even for such cases.
Concerning the last outstanding item c): we do not really expect many new particles in the energy range just between sqrt{s}=200-300 GeV, otherwise we would not always cry for higher energy and more luminosity. But even for maybe a few light supersymmetric particles and only in case we really need threshold scans, one would use the same strategy as for the Higgs: results at high precision measurements at high energy to outline if and how many energy steps might be required for the threshold scans. So, my guess, also no problems with the lumi reduction in the range of sqrt{s}=200-300 GeV concerning item c).
Last remark: It has been shown that it will be important to have high luminosity and an easy path to polarized beams at 500 GeV, to have optimal tools for the physics studies, to get systematics under control and to verify new physics properties etc. Also, concerning these aspects I believe the new undulator position providing a higher yield may be advantageous.

I wrote a 10 pages physics summary concerning the undulator position in times of the post-Snowmass ‘White Paper’, providing also physics references. In case there is any interest, I could easily forward it to you. The physics has not much changed since that time, so, the arguments and references should still be appropriate.
Best wishes, Gudi

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I see that Ewan already answered your question about the damping rings circumference; but just for completeness (and general information) see the spreadsheet posted on the Cornell Damping Rings Wiki site:

https://wiki.lepp.cornell.edu/ilc/pub/Public/DampingRings/WebHome/DampingRingsFillPatterns.xls

This shows the possible fill patterns (and timing scheme) for the "EDR" (should be "TDP") baseline lattice with circumference 6476.440 m. 

You can also see the corresponding data for the RDR lattice, which had circumference 6695 m. 
Tor spotted that the RDR circumference failed to satisfy a constraint on the bunch spacing in the main linac, arising from the subharmonic buncher in the electron source; but unfortunately this was noticed too late to make a correction for the RDR. 
This issue prompted re-examination of the circumference, to find a better solution.

The physics folks are hypersensitive to this type of stuff as you are aware.  Or maybe you consider this as being the first immunizing shot for them which they need anyways at some point <g>.

1. DR Circumference: As you point out I expect this is a typo. (Should be checked).

2. I think 200m is nothing more than a 'conservative guess' on how far you would have to be offset to clear the IR region. I tend to agree with you that this could be smaller (as small as possible? Are there other constraints?). These types of details need to be sketched out. Off the top of my head I think it could be less than 100m.

3. Dumps: the dumps are only offset by ~3m from, so I doubt there is any interference if they are on the same side as the DRs. Ewan's proposal for the connection tunnels/beamlines to the DRs are closer to the IR than the current dump locations. Andrei has a proposal for re-using the primary beam dump also as the commissioning/emergency dump which would require an additional tunnel (see his talks from Dubna); in this case clearly they would have to be on the other side to avoid interference (unless there is a clever way to reuse these tunnels in someway.) Again, these are things to be sketched out to some scale to see if they work.

4. From all your points, I suspect this one to be the topic of most discussion. The undulator length issue needs to be reviewed (I don't think Ewan was that exact in his distances). The energy range between 200-300 GeV Ecm is clearly impacted; in that sense, this scenario does affect the physics scope in the WWS document which prescribes only 1/gamma luminosity scaling over the full range. Should we immediately reject this proposal based on that? Or should we evaluate what the physics impact might be and see what the WWS think about it? If I remember correctly, when Tom Himel and I worked on the original post-Snowmass White Paper on this subject, the final decision to put the source at 150GeV was primarily taken on physics-scope grounds. All of the argument for and against are quire well documented, and it certainly warrants having that info reviewed. Something to do before the next WebEx meeting on the 18th July.