I personally don't think the port needs to be big enough to fit your hand inside. If you're going to use these tanks, you're doing yourself a disservice if you don't make yourself up a cheap pond pump CIP system. With the Atomic ABC or PBW or what ever percarb cleaner you prefer, I literally take my Snub Nose, pop it over the outlet upside down on my bucket blaster (the one from KL and KK are the same thing really) and let it clean for 10 minutes. I then rinse with the garden hose and bobs your uncle.
Crystal clean, no beer stone, no nothing. By having a large opening, you are only encouraging people who don't know better to stick their hands inside and clean it. This isn't needed, and will create micro abrasions for infections to live in. So, I don't see the point of a bigger opening. And if you look at the units which are having issues, they have big openings up the top. The larger they are, the weaker that point. It's why you don't see an airplane with floor to ceiling windows.
Just my opinion, and everyone is different, but if you want to design a product that is safe, and minimises the risks to people who don't know better, I think a smaller opening is the right move. The cone shape does indeed provide a better volume of clean beer at the end of ferment, however it comes at the price of being taller. As such, the rounder will suit some people, and the snub others.
Also, I don't think you should add a separate blow off port, as the vast majority of people who use these, ferment under pressure. This keeps the krausen down, and if you stick to the limits indicated on the fermenter, you won't need a blow off. If you do feel like you absolutely need one, then simply rigging a disconnect up to the gas post with a piece of beer line is a blow off done. One less opening / possible weak point.
In relation to producing them to a 'lower' pressure rating, I don't see the point. If it's rated to 2.4 bar or 35PSI and people want to run on the knife edge that's up to them. They also shouldn't undergo 'catastrophic failure' unless you have gone past the 2 year mark and have not had it pressure tested as it tells you to do ON THE FERMENTOR.
So, why create a product that is PET, light weight, with gas posts and liquid posts and then only construct it to handle low pressure? Doesn't make sense. It's like saying all cars should only have enough power to reach, and maintain 110kmh. As that is the upper limit. Why bother having more powerful cars? You don't need it, and people only end up driving at the end of the power range, and sliding their cars into gutters. It's not the cars fault, it's the driver.