How do I repair big cracks in a fibergl tub?

I have seen this before and there is a solution. First you need to know what the original problem is and how that can be fixed. When they told you that the tub was not laid down properly that was close to being correct but probably not completely so. The problem is that it was a cheap tub to begin with. The better tubs have a combination foam and fibergl base molded into them so that the amount of weight on them does not matter. Your tub clearly does not have that structure as indicated by the weak spots. this can be corrected in two ways. First , if possible gain access to the underside of the tub by cutting a hole in the sheet rock wall behind the tub just above the base board. The hole needs to be large enough to see into the area under the tub and then reach in with a can of spray foam and spray the whole can of foam into the space under the tub. This will take several cans of spray foam, maybe 3 or 4 or 5. When spraying the foam you have to reach in as far as you can and spray the foam in a manner that fills up the entire space under the tub. It is important to use a foam the expands as it cures. You may have to make a couple of holes in the sheet rock to gain the access you need. Now if you can't gain access to the area under the tub through a wall you have to drill several holes in the tub and spray the foam into the area under the tub to create the support structure you need. Obviously your repair guy will have to patch those holes. Frankly I think your repair guy was trying to do the repair on the cheap. I have seen this repair before and it usually costs more than $200.00. Your repair guy did as little as possible and said a prayer as he left your place that some miracle would happen and the leak would be stopped. As it stands he over charged you for what he did. Trying to fix it yourself is hard as you are not likely to be able to get the materials you need to do it well Fixing holes you drill is hard. The stuff they sell in the home center is not that great unless you are very experienced in using that stuff. There are companies who know how to do this well and if you can find one of those it is worth the money. If your original repair guy can't fix it get your money back. As far as being stressful get used to that and arguing. Make believe your married to the repair person and you just found out he made a p at your best friend.