Dood, I would at that point be tempted to break it more, and send it in if I got that response.
I have never, ever, ever heard that one was not supposed to wash every part of the tank, and honestly, unless there were a very large card inside in bold red lettering from the manufacturer stating "DO not WASH THIS TANK and also SURE, WE'LL PAY FOR ANY ILLNESS INCLUDING HEP C if you don't wash it," they are still kind of liable.
It's not as if people don't routinely wash tanks. Unless you can find explicit instructions ANYWHERE in your kit email them back saying a) I wash tanks because not doing so is gross and b) if your tank is too fragile to wash, it should be EXPLICITLY stated and c) WTH????
It's not as if sending an extra o-ring would be OH SO INCONVENIENT for them.
If have no idea if your tank is "unwashable" but I have not heard of the concept before it's pretty routine to wash tanks.
This is nuts. This tank cost you over 50 bucks, right? Fight it, join their FB page and complain. LOUDLY.
I had to do that with one of my warranties, but I eventually got it. I'm not normally that fierce, but it became a principle thing. I would up getting it.
Anna