Your poppet looks OK to me, but without seeing it first-hand, it's only a guess.
Make sure you are using a fresh O-ring on both the dip/gas tubes, and another fresh one on top of the post. Tighten the post reasonably tight but do not overtighten.
And you're be right. This is what I have in my garage at all times:
CO2. CO2-NO2 mix. CO2-Argon mix. Propane. Natural gas. Acetylene. O2. Gasoline. Diesel. Kerosene.
And that's just the gases(*)!
"Thinning" at 4 days? I assume that it's near the end of fermentation and not carbonated. Before determining if it's thin or not, you need to carbonate it at the desired volume.
If it's thin after carbonation, you need to review your mash temperature and increase the temp to increase body.
MC
Definitely water. I used to brew a fabulous hefe-weizen when I livedin Canada (near Montreal) but I've struggled to make a half-decent one since moving to the US (Columbia, SC).
in THIS case Topo Chico is sparkling water. But Coca-Cola bought them a while back and is selling hard seltzer under that name now. The fact that the bottle says 0 calories leads me to believe it's sparkling water and not hard seltzer.
This is the right way to do it. All pilsner except for a small amount of wheat for head retention.
Adding Munich malt will make it maltier and less 'crisp'.
All you need is about 30 ft-lbs on the CO2 regulator to tank connection. An 8" crescent would work in a pinch, seeing that captive CO2 wrenches are about 10" in length.
I'm by no means an expert, but I had decent results by using cream of tartar.
The final temperature determines the "crack level". The level of darkness was determined, in my case, by the number of water additions, so going from to cooler, to hot, repeatedly (be careful as sugar syrup can...