@Morrey is right, RO and distilled water are 100% interchangeable. Either is (nearly) devoid of minerals.
You add your minerals and acid to
both your mash and sparge water, before you heat them. Stir to dissolve.
That water calculator will help you figure out how much to add. Input your grain bill into their respective categories. It will calculate your acid deficit, and how much acid to add to get to your target pH (say, 5.35).
150ppm Cl- and 75ppm SO4-- is a good start for a NEIPA, as the originator of the recipe already stated. Use those numbers and his mineral addition tables, with weight amounts measured in (partial) teaspoons.
6a. No starter???
At minimum you should make a
vitality starter, shaken not stirred. Do it for 4 hours while you're brewing and things are cooling.
For such "shaken" starter use a gallon jug (no smaller) add your yeast and no more than 1-1.5 quart of 1.040 wort, cap securely and shake well for 30-60 seconds until very, very foamy. Loosen cap. Repeat every 20-30 minutes or so. Let some fresh air in each time before shaking. Pitch the whole lot.
One thing to remember, the wort coming from your mash tun is loaded with bacteria, lactobacillus mainly. Unless it's boiled, it will get sour within a day.
If you don't have any (spare) DME on hand, you can use some of the wort from your mash tun to prepare your vitality starter. But you need to boil it first, for 5 minutes, in a pot on the stove to make it sanitary. Cover and let cool to room temps. You can speed that up by sticking in a sink or tub with cold water. Then pour into the gallon jug and add the yeast. If you use a WYeast smack pack, smack 3-4 hours before opening or adding to your starter wort. It gives them a head start.
7. To aerate your NEIPA wort, you can use a (large) whisk. Good sanitation is paramount when handling cooled wort, beer, and yeast.
Or clamp that little white "spray cone," that came with your mash tun, to your racking hose, the end that's inside your fermentor, when filling it. That's is what that thingy is made for, to aerate your wort when racking, not to aerate your sparge water, which is a no-no.
8. What are you fermenting in?
Any temp control?
9. How are you sparging?