Is 0.68 static pressure too high for the ECM blower on my new two stage furnace?

0.68 on high fire with a clean filter is workable, but you're not sitting in the comfortable zone. The 1 inch rating in the manual is a ceiling, not a target, and the catch with an ECM is that it won't lose airflow as the filter loads up. It just spins harder to hold the same CFM, so the static creeps up and the motor runs hotter instead of you noticing weaker airflow like you would with an old PSC blower. That's why the headroom matters more than people think. Your low stage reading at 0.35 is genuinely fine, and if that's where it lives 90 percent of the time you're not in trouble today. But high fire kicks in on the coldest days, exactly when the motor is working its hardest, so that's the number to keep an eye on. Your balancing is almost certainly part of the number. Every register you close forces the same air through fewer openings, and that pressure has to show up somewhere. I'd start with the two basement registers since those were just extra dump zones anyway, no reason to restrict them. Then back the pinched upstairs dampers off a little at a time. Mark the positions with a sharpie before you touch anything so you can get back to where you were. Then run high fire and re-check the reading to see how much you bought back. On the filter question, don't guess how much it'll climb, just watch it. Check the diagnostic once a month as the filter ages. If you're at 0.68 clean and it's pushing 0.8 a few months in, you've learned to swap it early instead of waiting out the full season. Some filters gain pressure fast once they start loading. The CFM dial-back in the thermostat does work as a pressure relief valve, but it raises supply temps and can shorten cycles, so I'd treat that as plan B after you've opened up the ductwork. Try the dampers first, since that's fixing the actual restriction instead of masking it.

Been through this exact balancing headache on my own setup. What finally made it manageable was a cheap digital manometer, the kind that comes with the little tubing for taking duct readings. I'd tweak one damper a quarter turn, run high fire, write the number down, tweak the next one. Took an afternoon but I coud actually see wich register was buying me the most relief instead of guessing. Ended up finding one return that was undersized for the whole system, which was the real culprit. Might be worth checking if yours has a similar bottleneck before you chase dampers one by one.

One thing I learned on mine, opening the dampers only got me so far. I was chasing the supply side for weeks before I poked around the return. Mine had one undersized return grille pulling through the whole house and that was half my problem. Added a second return opening and the number dropped way more than any damper fiddling did. So if the registers only buy you a little relief, look up the return path before you settle. Also on filters, I switched to one of the lower restriction pleated ones and it made a real difference at high fire compared to the fancy allergy filter the installer left in there.