You may have a fungal or bacterial problem which may be environmental-in soil or airborne. For general health, it helps if your brassicas are planted in an area that will get a lot of sun and the drainage should be good. They like a lot of organic matter in the soil so that it isn't too dense with clay.

Sodium is not salt. Salt is sodium chloride. During softener regeneration sodium ions are exchanged with calcium ions but the chloride part of the sodium chloride that makes it a SALT goes to drain. There is no salt in softened water. By rust do you mean a white crusty deposit or brown RUST like when a tool is left in the rain? Posting a picture of the rust would help us see what you're seeing. If you scrape off some of the rust(?) and put it in a dish and add a little white vinegar or LimeAway does it fizz?

Based on what you described: Do not apply anything unless you know what the problem is. (Btw, where does vegetable oil idea comes from? Just wondering - didn't come across that recommendation yet) Chilly windowpane is no problem - unless leaves/part of plant is touching the glass, and there is frost on it. Or really drafty... In z5, I have them on windowsill a do not worry about chill.. Cooler temps during nighttime are often quite suitable for many plants. Watering 1x/week is probably too often, and even more so if the potting soil doesn't drain fast. I can see some perlite, but can't tell what is the mix made off - do you know? How warm is it in the room they are kept? As for the white spots, are they web-like? If so, it may be mealy bugs. But I really can't see detail well enough to be sure. Does it look/feel papery-like? The larger cacti may have rot problem - I never had to do surgery on something similar, so hope someone else will post with good tips. As for light, they need as much as possible - many of us use lots of supplemental lighting during dark & short days of winter. It is a good idea to isolate new or diseased plants from the rest, usually until there is no sign of disease/bugs anymore.

Oh dear. Technically the store is the point of sale. If the store won't help you, you need them to put that in writing. Normally manufacturers have a relationship with the retailer and the retailer does all the leg work to notify the manufacturer there is a problem. I have no idea what has already been said to Karndean so please take this with a grain of salt if you have already gone through these steps. First off, Karndean has to back track to find out when your retailer ordered this floor. The retailer is the one to deal with this type of stuff. Normally you contact the store, have someone come out. They document the issue (time frame, purchase date, installation date, etc) and then they send off the warranty complaint to Karndean. That's the normal way of doing things. If the retailer has stepped back, you need to get that in writing and then send that in writing to Karndean...along with ALL of your documentation (purchase order, purchase date, item numbers, etc) found on a detailed sales receipt (ahem...from the retailer). Whew. Now that has been done, it is up to Karndean to go back and forth with the retailer. But wait. Your retailer doesn't want to do this. Ok. Now they go back and forth with you...and your local Sales Rep. Ahhhhh....therre's the rub. The 'Rep' is a SALES rep. Not a technical rep. Oh dear. Now it will take LONGER to deal with this. Hmmm. Sigh...I know this is discouraging. I get it. Hang in there. Send Karndean (or your rep...whichever) all the documents you have of this floor. The sales receipt and the statement from the retailer that they refuse to deal with this issue (that's rare...because they had a contract with Karndean at the TIME you purchased...which means they normally have to hold the bag for the duration of your floor's lifetime). Do you have any boxes left over? Normally the retailer will ensure you purchase 10% extra so that you have enough waste to complete the job AND have 5% left over for future repairs. Did the retailer LEAVE your boxes behind??? If you paid for them they belong to you. Which is why you should have at least a few planks left over (I hope). It is entirely possible the floor no longer exists and that a patch is going to come from your own home. Example: if this floor runs through several rooms, the person doing the repair would pull planks from one room (like a closet) and use them to repair the (?kitchen?) floor. How old is the floor? Would you say you have MORE than 10% of the surface is damaged throughout the space? Or would you say there is LESS than 10% surface damage? If you have 1000sf of this stuff (just to make the math easier) you MUST HAVE 100sf of damaged floor surface (not planks with spots on them but FULL removal of the surface over 100 square feet) for a 'wear through warranty' to kick in. This is a common disclaimer made by many flooring producers. If you have access to the warranty you purchased your floors under, you will need to look up the specifics of it. I may be wrong about the 10% surface damage...Karndean may have a different warranty than other vinyl producers. That's why it is critical you find the warranty document (written for the year you purchased) AND you take a educated guess as to HOW MUCH of the surface has wear through damage.