Sorry that happened to you; new tanks can be very frustrating and this looks like a classic case of NTS (New Tank Syndrome).
First off the cloudy water you experience early on was a bacterial bloom caused by the nitrogen cycle. This forum has a nice sticky that explains Aquarium Cycling that you may want to look at. When starting a new aquarium, no matter the size, it's very important to get a *few* hardy fish (Zebra Danio are a favorite, though White Cloud Minnows and Platies are sometimes used). Female Betta are not known for being hardy and tend to die off easily (at least in what I've seen). Platies are schooling fish, as are Mollies; they need to be kept in a school of at least three or they will become stressed very easily. The stress of being alone in addition to the near toxic water of a cycling aquarium is probably what killed them off. Mollies are too big for a 10 gallon tank. They usually get about 4 inches in length and are known for being semi-aggressive when they don't have enough space.
As for what happened to your fish that appeared to be swim bladder- I'm thinking it was more likely Ammonia burn. With swim bladder they tend to literally do flips in the water and swim almost vertically. Ammonia burn/poisoning affects the gills and the fish will struggle to stay at the top, usually trying to get air. This is why it's important to do small water changes throughout the first few months of setting up an aquarium; it keeps the ammonia/nitrites down to safer levels. What did Petco test for? Ammonia is crucial for testing in the early days of a tank's cycle and pH can play a big role too, especially since your pH effects how toxic the ammonia is to the fish. Extremes in pH in either direction (below 6.5 or above 7.5) can really cause problems. I'm assuming you got new fish right away, which didn't leave enough time for the bacteria to die off in your tank, resulting in your tank finally cycling through
Now for the next thing: NEVER clean out your tank. The only reason you should ever clean out a tank is if there's a nasty parasite and everything dies and you want to start out new. Small water changes and gravel vacuuming is important, but never totally clean the tank. Fish do no live in sterile environments and actually rely on the bacteria that grows in their tank. This also goes for filters. When you do small water changes you can take your filter cartridge out and rinse it with the water you removed, but not under tap water, as that will kill the bacteria. Once you've gotten some of the "gunk" off the cartridge, just put it back in. A lot of your bacteria lives in that cartridge and you don't want to kill it off. Filters should be changed every 2-3 months, despite what the packaging says (they just want to sell you more supplies).
I'm glad things are getting better for the tank. You'll want to keep a close eye on your Male Betta and the guppies, especially if the gups are males. The fancy tails on Guppies look like other Bettas to your Betta and he will attack them. Usually it isn't good to mix Bettas with Guppies, but it does work in some cases. You may want to rethink the Molly; as I said before, they really are too large for a 10 gallon and can be aggressive. Bettas and Guppies are both slow-moving because of their long fins and thus look like tasty nipping snacks larger and more aggressive fish like Mollies.