There are probably a near infinite number of ways to achieve what you want to do. As you've probably gathered from this thread, some of those ways can be very dangerous. Always proceed with caution. First Rule: never have ANY closable valve between the tank and your PRV. If at all possible, always have some sort of secondary PRV that is NOT capable of being shut off any time you are pressurized.
If I were configuring your setup to achieve what you're attempting, I'd first get a 4" TC to 1.5" TC reducer and mount the PRV shown in the first picture and mount it on the 4" TC port on your fermenter. Leave it there ALL the time, except for cleaning. Seriously. That device is both a PRV and a vacuum breaker. It's not adjustable, so it can't be Murphy'd up. Make sure you buy one that has a release pressure that will open at a pressure less than your tank's rated maximum pressure. For instance, a 15 psig rated unitank will likely have a maximum rated (tested) 30 psig pressure. You never want to actually reach this pressure, but if you do you'll survive the episode. That's why you always want it in place. For a 15 psig normal operating tank, look for one that releases around 22-24 psi. They're not terribly accurate (that's why they shouldn't be used for spunding) but they're very reliable.
The vacuum breaking feature will also keep your tank from imploding. Pressure tanks can hold an amazing amount of positive pressure, but will fold like a cheap suit at amazingly low vacuum pressures. That device in Picture #1 protects you both ways from Mr. Murphy and any unintended ***ups on your part. Everybody makes mistakes from time to time. This devices keeps you from making one that could kill you. Never "valve" it, always use it. No exceptions.
The manifold in Picture #3 is a really neat accessory. I've been lusting after one for quite some time, but Santa didn't leave one in my stocking this year, so I guess I'll have to take matters into my own hands. The PRV in the manifold is supposed to release in the vicinity of 15 psig, and maybe it will if it doesn't clog with krausen. Don't bet your tank on it. It has no negative pressure relief. It's adequate as a secondary backup relief valve for positive pressure, but I wouldn't trust it for primary /singular protection. It does give you a quick and reliably dependable way to depressurize the tank however. The spunding valve in Picture #2 is an adequate, but not optimal, way to spund. But NOT at the beginning of fermentation. Yes, it is capable of releasing an overpressure, but the very fact that it is adjustable makes it unsafe to use in place of a PRV (or two). I have two of them and often use the with kegs, but only after having set and verified the pressure release point by measuring on a pressurized keg that has greater pressure than what I want set on the spunding valve. The keg also had a built-in PRV. It's also rated to 130 psig. (PRV releases well below that pressure).
If you intend to spund with the device in Picture #2 wait until the fermentation has reached high krausen and is subsiding, preferably when the wort is approximately 5 points from Final Gravity. But first cycle the PRV on the manifold to make sure it vents and hasn't gotten clogged with krausen. Krausening this way is pretty safe as long as you do these steps and wait until high krausen is subsiding. It's a great way to carbonate beer.
So, good to go, right? NO.
We never discussed the initial part of fermentation. You know, the part where the vast majority of CO2 is being generated and the risk of overpressure in your tank is the greatest. Here's two possible ways to get where you want to be, and they both involve a blowout line just like the one that came with your tank in the first place. The first way is to start with the ⅜" barb x 1.5" TC cap. Attach it to the 1.5" TC fitting on the top of the tank. Set a ⅜" silicone hose line on the end into a jar of sanitizer. Let fermentation happen. When you're SURE that high krausen has passed and the fermentation is slowing down, swap the blow-off line with the manifold on the tank-mounted 1.5" TC port. Closely (and I do mean closely) monitor the pressure to make sure the spunding valve is properly set and releases at your desired pressure.
Another way to skin this cat would be to mount a T pipe on the tank's top TC port and attach both the blow-off line and the manifold to it. Start fermentation with the blow-off line in place. When fermentation slows, swap the blow-off TC fitting with a 1.5" TC end cap. If you're quick, the ingress of oxygen will be minimal and will likely be consumed by the ongoing, but slowed, fermentation.
TL;DR. Bottom line: you NEED a dedicated PRV, unadjustable, not able to be isolated from either the tank or the outside, preferably with negative pressure relief capability, in use AT ALL TIMES! You need a dedicated blow-off line to vent the huge volume of CO2 being generated in the early stage of fermentation, that you can replace with a true spunding valve once fermentation begins to slow down.
Pressurized fluids can be enormously dangerous. Multiple pressure relief pathways are desirable. Pay close attention and take nothing for granted.
Spunding is a great way to carb beer naturally while saving money on CO2 tank refills and cutting a week out of your time between grain to glass. I'm not convinced specifically that fermentation under pressure has much net advantage on the homebrew scale except possibly for lagers. I've tried it a couple of times and really saw no direct benefit. Spunding on the other hand is very worthwhile and is the thing that makes pressurized unitanks and pressure capable fermenters worth having.
