mints can do better in the shade with little direct sun. I am in zone 8, temps hovering near 100F now and my mints are doing ok, not as good as spring time though. I would suggest that you move your mints to a mostly shady location and water them more frequently. But no matter what you do, they will no be as tender as they used to be in the sprng

Although there is a huge crowd (mostly people who are on the whole organic gardening train), that believe nutrients need to be in a chelated form to be absorbed into plants. Although that is COMPLETELY correct, their mindset is that rust will stay as rust, but that is completely false. An example people use today is ammonium. Ammonium CANNOT be used by plants in the least bit. It is however converted into nitrates through microorganisms. The same goes for rust. Rust (or Iron III oxide), will either end up chelating to organic acids in the soils (such as humic, fulvic, or acetic). It is there where the oxygen of the rust will combine with the hydrogen in the acids to form water, and the newly freed Iron III ion floats around dissolved in water until is forms an insoluble precipitate via chelation (most likely with the conjugate base that supplied the original hydrogen). It is in this form that it can absorb through the roots. This process is simply accelerated by microorganisms and highly acidic soils. But it is complete BS to say that rusty nails won't help with iron deficient soils, it will just take a lot more time for it to work. And if a plant is already dying from iron deficiency, then it's most likely going to die before it starts absorbing some. That is why it's important to give a dying plant immediate attention with a EDTA or amino chelated form of iron. Or You can dissolve nails (Or better yet, steel wool) in vinegar. You can enjoy this process first hand by placing a fire-stick above the soaking wool and bumping the glass to have the hydrogen bubbles float to the top all at once. It makes cool popping noises. However, your suggestion of Iron Sulfate is no different from rusty nails soaking in sulfuric acid.

Water slowly, and use your finger to check the depth of the moisture -- and it's going to require lots more water than you expect! Drip irrigation (even in the form of a tiny hole in a bucket's bottom) is far more effective than a sprinkler, which loses too much to evaporation. Poking along the dripline, you should be able to feel moist soil at least 3" deep the day following the watering. Also, a deep mulch will help prevent evaporation in those hot temperatures. Link is to one sort of drip irrigator, especially handy if you don't like to poke holes in your buckets! Here is a link that might be useful: LV's tree irrigator

Any damage that is showing on your plants is mechanical - that means it came to be from shipping and other transport, handling or jostling. It is not the result of diseases or insects nor does it look like any watering issues. In reality, these plants look pretty darn good considering most big box stores do not take the slightest bit of care with their plants (hence the evidence of damage you are seeing). As to fertilizing, you need to identify your plants and then do a bit of research, as not all plants benefit from similar fertilization methods nor do they require it at the same time. Cacti and succulents have different needs than do tropical foliage plants and flowering houseplants (orchids, African violets, flowering gift plants) have yet another set of needs. In general, an all purpose liquid fertilizer used at 1/2 to 1/3 strength with every other watering is appropriate for most tropicals during their primary growing season (March through October) but you can get by with quite a bit less. I'd suggest you post this on the House Plants forum both to get accurate ID's on your plants and their advice as to when, how much and with what to best fertilize what you have.