Thats a lot of water to be taking on. Definitely more than just one or two rivets leaking. Prior to my resto and temporarily taking out bad solids for blinds, I was taking on that amount of water with 40+ bad rivets. My boat had hit a log and one side basically broke half of its rivets due to the impact. The temporary blind rivets and sealant actually got the leaking down to a manageable 1 gallon a day.
My first thought for you is the bolts that hold the motor on, may not be sealed well. If the boat sits on bunks, maybe the bunks are pinching the leaking spot when you fill her with water to test. Or you may have a lot more bad rivets than you think, but if that were true, you would think the leak test would show something. Some of the bad ones I had looked good but the heads popped off easy because they were compromised. Hence the sinking story earlier…
But honestly at this point, with that bad of a leak, its safe to say she needs to come apart. You just trying to get a summer of fun before fixing it right? I understand that cause I did it.