You sure like to call me out on everything don't you John....
For blocking the port I'd hardly call a soft stop on air harsher than hitting the breech which would send a shockwave up the spring.
The heavy pellet this goes like this (in my mind). And keep in mind that this varies on the guns design. The heavier pellet holds the pressure longer reducing piston impact on the breech. If I have a freshly oiled gun the dieseling is less violent with heavy pellets, the heavier they are the greater the effect. It will diesel for more shots since each firing is less powerful, but better to have 5 low powered than 2-3 violent ones. Now, once violent (loud) dieseling has stopped with heavy pellets I can shoot a light pellet and it will diesel again which tells me the pressure was higher than with the heavy. I don't believe this has to do with port pressure, but pressure at the piston face which increases dramatically the closer the piston is to the breech or the force at which it hits. Lighter pellets allow the piston to hit harder therefore they diesel more. If the dieseling were in the transfer port alone then yes a heavier pellet would be worse, but if it were in the port alone I doubt it would be a problem. Another item to show dieseling is all about the piston face and not the pellet, you can take a gun that has stopped dieseling with light pellets, dry fire it and it will diesel again. It does so because you sent pressure at the piston face even higher than before because it hit harder. Dieseling in the port would be impossible without a pellet. All this tells me that piston impact is what causes dieseling just like clapping your hands harder = louder. Another point is guns that I've increased the compression ratio diesel less. I know that doesn't sound right, but it reduces piston impact and thus hot spots that ignite the oil, kinda like how a poorly designed cyl head in a car will ping or diesel before a well designed one will, even when the better head is higher compression. It's not the same principles, but being designed better is the key.
Now like I mentioned before it's all about the design of the gun. The relation between the bore and stroke is a factor, the smaller the bore the more likely it will diesel, but it's not as harsh on the spring when it does. Piston speed is a factor, a heavier spring increasing piston speed also increases dieseling. If you have a gun like a cheap chinese model with a large diameter but very short port then a heavier pellet will cause a bump in pressure and also put more dieseling pressure on the piston. And due to the small volume of the port the pellet starts moving sooner in those cheap guns than with a modern gun so pellet weight is an even bigger factor because the lighter pellet has moved sooner thus decreasing pressure. Since those cheap guns don't usually have nearly the power, and no real volume to the port it makes all the difference and the two guns cannot be compared.
If you have a gun where the piston doesn't hit the breech then that too will change things and the lighter pellet will be slightly better at reducing dieseling (in my mind). But since most all efficient springers have pistons that hit the breech, and most all guns sold are like that, then my idea of what's happening covers most all modern springers. That's my story and I'm stickin to it
I urge you to experiment, including plugging the barrel to simulate the heaviest pellet.