nitrate readings low -- what to do? and a few other newbie plant ?s

It's too early in the morning to get too detailed but I don't think the ph shift with the DIY will hurt your fish. My change after a water change is probably as much and no problems over the last several months. Keep the plants growing seems to be the secret. If they aren;t, then the algae take over instead.

Until you add some sort of fertilzation using CO2 gas, don't worry about the NO3. 5ppm of NO3 is fine. But there's a great deal of problems associated with NO3 test kits, for the most part they are inaccurate in the ranges that plant tanks need.
Lamott test kits are not but they cost 60$ or so.

You have a fair amount of light on a 55gal. Your tank is not limited from light in any way, nor likely nutrients but CO2 you most certainly are.

Plants need 3 things, light, CO2 and nutrients.
In low light and non CO2 tanks you can get away with not adding CO2 since the growth rates are slow but you also need far more patience, less plant choices etc.

As you increase the light, you need more and more CO2 to supply the plant's needs.

*Adding CO2 at low light really improves growth a great deal also which many many many people forget and don't seem to understand very well at all. You the benefits of both approaches with few of the problems.

As far as you tap water, KH of 5 and a GH of 12 is excellent. Soft water does not grow plants anymore than hardwater. Plant simply care about CO2, nutrients and light. They don't care if the water is hard or soft.

As far as your pH going from 8.0 to 7.2 in a few hours: This happens in nature in even larger amounts in shallow lakes. Daily fluxes of 2-3 whole pH units are common.

You will need to knock the pH down using only CO2 gas to 6.7 to 6.8 and keep it there for the entire lighting cycle for you KH of 5.
That's all you do for that. No monkeying with RO, DI, distilled water, worrying about pH "swings" etc.

For NO3: Add 1/4 teaspoon each time will give an extra 4~5ppm of NO3 to your tank. Until you add the CO2 and get the pH is a decent range, don't mess with this. Once you do get the pH down, then you can add this amount about 2-3x a week. Also get some K2SO4 and trace mix like Flourish etc and some KH2PO4(all can be had at : www.litemanu.com).

CO2 is the main issue right now though.
Regards,
Tom Barr