1. You are experiencing a problem common to all CLCs in that the rated pump flow is on;y a fcartion of a gpm.... for custom loops, the minimum is considered 1,0 to 1.25 ... more doesn't help but less does. And that rated 0.3 is w/o all the extra tubing, fittings for the GPU so it's far less than the rated flow.
2. Marketing types love to play with numbers. When a package reads 0.30 gpm / 2.0 psi that doesn't mean what it says ... it means it can produce 0.30 gpm @ 0 plsi and 0 gpm at 2 psi in marketing speak. So the reality is you are likely down around 0.1 to 0.15 gpm at best.
3.a If I understood correctly, you have a Open Loop type AIO with a GFX card cooler designed to be connected together and using 2 pumps. I don't understand where / how the NH-D15 fits in. What type of bleed mechanism did you employ and what procedure did you employ to remove air and "off gasses" ? Without doing so, there will be air trapped in the system which will inhibit flow. In a custom loop, the best method is to install a rider on one of the spare rad ports on the top side of the top rad. If the rad has no spre ports, one mist install a tee at the highest point in the loop with the branch side pointing up. Here one can attach a valve and a rigid acrylic tube with valve on top. Tilt the system as necessary to have the tee at the highest point, fill the tube 90% of the way and close the valve . Typically as the fluid traverses the loop, air / gas will come out of solution or any bubbles that would have formed will collect on the tube and you will have a visible drop in the water level in the tube. Shut down the system, open the valve and top off as necessary, rinse and repeat. Not that any high points in the system may have trapped air.... you will need to work these out by bending tubing or tilting the system.
3.b Not using the 2nd pump presents a significant flow restrictiion as pump 1 is being forced to spin the 2nd's pump and motor
4. While not optimal, there's nothing terribly wrong with your application method. However, introducing finger oils to the TIM is a bad idea. You can use a plastic glove ot some TIMs (i.e. Gelid) come with an application "spatula", However once spread around (smoothly spread method came in 2nd place in 2nd link) , use a plastic credit or hotel lock card to scrape off the excess, leaving behind only the stuff that is left in nooks and crannies. This method is often used on HDT type coolers after the two line application method is used. What method you use depends upon the cooler / block type.
https://archive.benchmarkreviews.com/index.php?option=com_content&task=view&id=170&Itemid=4
https://www.pugetsystems.com/labs/articles/Thermal-Paste-Application-Techniques-170/
5. Note that many blocks are designed to prevent overtightening by including a "stop" mechanism ... you screw them down till you can't go any more w/o abnormal pressure. Try and go beyond this point and you will strip or break the screw. It also allows for absolutely even contact pressure.
6. Other than Ivy Bridge, I have not encountered an Intel CPU that requires delidding to reach the highest OC using custom water cooling or OLC type AIOs.
I don't see this being resolved w/o addressing the problems above . I know money is tight, but ultimately there are a whole lot of deficiencies to overcome. While I applaud AC for producing a AIO w/ a copper rad, going cheap on the pump was abad idea. Yes it saves money but that is immediately offset when you have to add a pump for the GFX card. AFAIK, Swiftech and EK are making the only AIOs that both provide all copper systems (not sure about the GFX system) and provide a pump with the necessary flow capacity to adequately do water cooling. The EK system is the only one that doesn't (theoretically) require bleeding.