That seal looks like a standard spring-assisted cup seal in the drawing, so you just hook it out with a small hook probe, or (worst case) you poke a hole in the metal shell with a sharp prick punch, drive in a sheet metal screw and yank it out. However, if it isn't leaking so much that it is costing you money to replace what it leaks, then I'd ignore it until it gets really bad - the leak is flushing any dirt out, so no damage will occur. Having to cut one of those seals to remove it is very common, by the way. At the factory they are often installed with a bit of sealant or shellac on the edge to seal them and that sets up like glue. When you replace the seals, if they have a bare metal rim it should be coated with a sealant prior to installation. Some new ones have a sealing coating applied at the factory and some don't. I always seal them in any case, so they don't leak and can't spin in the bore. If yours went in too easily they may leak around the outside unless sealed with a coating.
As for overfilling your lift box sump, that could be a problem, but NOT if you have screwed in the dipstick. Everyone seems to say that if you do that you will end up overfilling the sump, but actually the opposite is true. If you think about it, screwing in the dipstick puts the line on it lower than if you just rest it on the cover. That would mean that your fluid would reach the line at a lower level than if you simply rest the plug on the cover, which puts the line higher than it is when screwed in. Not meaning to jump on Chris, but I've seen this same statement so many times that it drives me nuts and I finally had to say something.