There are some photos here... The spores are bright orange, not yellow and very powdery. If you look at the photos you can actually see little raised bumps. Every fall I go bargin hunting at the local nurseries looking for prennials I refuse to pay the full price for and a couple of years ago almost every Daylily they had was covered with rust, so yes nurseries can be a source of rust. If you decide to speay make sure you use a systemic. The first time I had it here I used a contact spray which looked like it was working but the rust kept coming back. I eventually gave up and let it run it's course. I am in Zone 5 so winter kills it. I had it in 2005, 2006, and 2007. I have since changed my buying habits and have not seen it again. I no longer buy from people in the south or areas without the extended cold spells it takes to kill it. If they buy new intros from the south I get them to send my plants out as early as possible to avoid their new plants sending it here. It is unsightly but I cannot say it has actually killed any plants here. I am sure it set some back and they did not preform as well the following years. Leaf streak can look like rust, look for the little RAISED bumps and the orange powdery spores. I was out in the garden this morning and saw 1 case of leaf streak that made me check to make sure that's what it was so even though you have seen rust a quick view of any miscolored leaves tends to make you think about it. It is not the end of the world either. Here is a link that might be useful: Rust

It is a huge design fail to mix irrigation types for the same zone. If your rotors/sprays have output adjustment (ie change nozzles, spray heads) you can mitigate the effects, but it's still a bad idea.

RoundUp (glyphosate, but RoundUp actually does include diquat dibromide as well for a speedier response as glyphosate is not a fast herbicide) deactivates upon soil contact and then decays via bacterial action (bacteria consider it dinner) over some months. I can't find any indication that it has any other rapid decay mechanism (and, in fact, most sources indicate that it's fairly stable in sunlight and very stable in water. Which makes sense; that's why the bottle is white and the listed dilutant is water. Personally, I'd rinse off the rocks (and the hose) with the hose a bit before interacting overmuch with them and I'd probably be hesitant to lick the rocks before a good rainfall. As far as immediate toxicity goes, there's little to no concern. Toxicity is >5,000 mg/kg in rats, which is pretty much going to be the same in humans. You won't be exposed to that much, and that makes it less toxic than table salt. And again, I'll encourage you to research the cancer links on your own. Personally, I consider the evidence fairly weak. Inevitably, somebody out there will come back with, "but I know somebody..." That's too bad...but "anecdote" does not equal "datum."

I see a porch on the front of your home and a deck on the side. A porch is a covered shelter in front of an entrance. A deck is an open wooden extension to a house. I was wondering if you wanted to add a deck to the front of your home, or redo the porch and railing. The posts for your porch are load-bearing as far as I can see. They cannot be cut off and capped. But perhaps you want to extend your porch out from under the overhang, in which case that part would be a deck, and you could cap those posts. Because decks are exposed to rain there is a gap left between deck boards so rain can drip through. Most means of hiding the area beneath a deck is therefore open to allow moisture to dissipate and prevent rots. The photo below shows an open horizontal board enclosure and also uses cable railing. You could, though, use the same rock as is on the front of your garage to close in the under deck area as long as sufficient venting is provided. I added this to your existing porch below. This would give visual continuity to the design as well.