Where did you get it/brand? You can try and do a fill test in the sink. Some of the cheaper Aqueon ones have this/a messy silicone job and I have had one with no problem. And yes you can always reseal it.
You can always reseal the silicone pretty easily...just buy a tube and smear it along the crevices where the glass panels come together...it's no big deal
Those joints aren't flush, though, and it looks like a few of the panels have gaps between the bottom of their edges and the table. That doesn't seem like it would be great for stability.
As David said. Why aren’t the sides flush with the bottom? This looks like a terrible reseal job. You could reseal it but I would move on. You want a strong intact tank especially for how long you plan to keep it.
Very thanks. I don't know which floor is crooked. Is it the floor of the dresser, the floor of the house or the floor of the aquarium. I'm out right now. I'll try harder when I get home.