Good job, you've got a good handle on this. You know what your issue is and it's backlash. Not any particular setting in PHD2. Your settings are good, you're getting sub arc second in RA and almost that in DEC, despite the moderate amount of backlash. Don't despair I've seen way bigger backlash that other's have fixed, heck mine was over 10000ms when I first got it and it's now at about 1000ms. Which I think is pretty common for this mount, others have got it down to the mid hundreds but I think that's about as good is you could get it. PHD can easily deal with 1000 - 2000ms of backlash with backlash compensation enable. As far as enabling backlash comp: you could try with my settings (remember those are optimized for my amount of backlash) and see if it helps. But, your backlash is beyond the amount that PHD BL comp can effectively deal with. GA doesn't even recommended using it, only guiding in one direction.
Your best option is to try to continue to get that DEC belt tensioned properly. The bad news is (at least for this mount) that's it's kind of an "art" to get that tension right. There's a fine line between too loose and too tight. Your options, as I see them are:
1). Continue to follow the iOptron belt tensioning procedure experimenting with different amounts until you get something acceptable (might be time consuming until you find the right adjustment). Do you have anybody nearby that has some more experience with the mount that could help you?
2). Send it back to iOptron for adjustment. They could upgrade the firmware so the motors put out higher torque too, just like they did for me. At least you know it should come back right, but that may be time consuming too depending on where you live. I had mine back in less than a week but I live near Chicago so not too far to the Boston area. Mine was under warranty but even if yours isn't their prices are very reasonable.
3). Not sure how you acquired the mount, but if you got it new from a retailer, talk to them and see if they'll exchange it. Beware the mounts, while overall are good, they do come from the factory with varying degrees of QC. So you may end up with one that has better, worse, the same amount of BL, or even other issues.
If it were me, I'd continue to try to adjust it or send it back for adjustment. You have a know quantity with what you have and it appears that you don't have any other real issues other than backlash (very common and normally very fixable). And just as Paul said make sure you're well balanced in DEC. How much weight do you have on your mount?
Your calibration looked excellent and your PHD settings look like they'll give you very good results once you get your backlash fixed (or at least more manageable).
Don't worry about the Period Length changing. That will automatically adjust as PPEC learns the worm period. It should settle around 598ms once you get a few longer guiding runs under your belt.
Good luck, and keep us posted.
PS - I'm by no means an expert on any of this. More of a novice really. Others my have better recommendations.