The damage to that oil seal appears to be due to improper installation of the seal itself. The tightness of the impeller has no effect on that oil seal. Do not just thread on the impeller hand tight, you will simply create another leak, this time of coolant.
When installing that oil seal, make sure it is bottomed out in the plate. It's very possible it wasn't, so when you put that plate on and then tightened down the whole assembly, the end of the seal got pushed around and distorted. Also, make sure the flat side of the seal is facing toward the plate you install it in. If you installed it with the dished part facing toward the plate, that could definitely has caused your problem.
The gaskets look reusable, you should be fine. And, as mentioned, make sure you replace the two o-rings on the pipe which connects the water pump to the cylinders.
Also, that big flat gasket which goes between the block and the impeller backing plate doesn't have much in the way of contact surfaces, so that's why it appears to be deformed. That's just the way it is, don't worry about it. The gasket between the impeller backing plate and the pump cover is the important one. Make damn sure both surfaces are scraped completely clean.
1990 EX500 - 141,000 miles as of June 30th, 2019
1993 Ducati 900SS - The real canyon carver
2002 EX500 - Undergoing a full restoration