Help - What is the Bottom Line with Nitrates? oh, and some general DE advice, perhaps?

[new to pools guy]

New to this forum. I initially sent a PM to chem geek about below, but realized I should probably post it more generally.
Chem geek, if you see this, great!

OK, so this is my first season with a pool.

Leslie's guys are telling me I need to drain my whole pool because I have nitrates and this is causing chlorine lock (this after putting several hundreds of dollars worth of chem into the pool).
My total chlorine is fine, but I can't get a free chlorine reading.

Pool looks crystal clear.

From my readings online, the Leslie advice regarding nitrate removal is suspect.

I am reaching out to you because you actually sound like you k now what you are talking about.

My phosphates were also very high and I have gotten them down to 600 from 2900. So making progress there.

But my real questions are as follows:
1) How do I free up the chlorine? No-cholorine oxidizer shocks? Hyperchlorinate?
2) If I have no free chlorine, is it safe for my kids to swim if I just shock the pool with oxidizer shocks 30 min before they swim?
3) Do I really need to get rid of the nitrates at all?
4) Should I wait on raising further my CYA until I fix my free chlorine issue? (CYA was close to zero, and I started adding a few pounds before thinking that maybe I should wait until I fix my free chlorine issue), or do rather need to raise my CYA so that the sun doe not eat my free chlorine? Or does that not make any sense?

Separate issue that noone can give me a clear answer on regarding DE.
During the nightmare that was using noPhos to bring my phosphates down, I had trouble keeping the PSI down on my filter. As a result I was having to do a quick backwash every so often to lower the PSI. Every time I did the "quick" backwash, I added a bit of DE back, but not the full amount. How are you supposed to know when it is time to open up the filter and manually clean the filter? I feel like I am playing guesswork every time I do a DE reload after a backwash and I have no idea how much DE is in there or not. The guy who opened my pool said I should use a little bit less DE every time I backwash until after a couple of months I should manually clean the filter...but during the noPhos extravaganza, I must have backwashed to some extent 5 times over the course of 5 days.

Thank you!

[Denniswiseman]

Nitrates are food for algae, dilution by partial drain and refill is the answer
If you have no free chlorine then you need to Slam (Shock Level and Maintain) with relation to Chlorine / CYA Chart and Recommended Pool Levels
Pool Maths
You can safely swim with your chlorine up to slam level for your cya

Test strips are commonly called guess strips
You really need to get yourself a decent FAS/DPD test kit (Taylor K2006c or TF Testkits TF100 in the states) to get accurate results as maintaining an appropiate shock level means testing quite often during the day
Regarding your DE I would be inclined to carry on as you are untill the pool is balanced and then do a proper clean of the filter

Use these common products to balance your pool
Liquid chlorine (sodium hypochlorite or plain bleach)
Muriatic acid (hydrochloric acid) to lower pH and TA
Bicarbonate of soda to raise TA
Aeration will raise pH only
Soda ash will raise pH and TA