It's true that you can't make an ID from just the flower, however there aren't that many yellow composites blooming this early in the year around here. I'll bet it's one of the Groundsels (Packera spp, they just changed the name recently so it's listed as Senecio in older books.) Scroll down for some pics very like yours. There are a bunch of different species, so yours might not be this exact one. WW Here is a link that might be useful: Ragwort, Packera (Senecio)

As below Campis sucker. Here is a link that might be useful:

I just saw the leaf furthest to the right in the last pic - definitely HVX. :( So sorry - it's so lovely otherwise - that stinks!

Yes, I think so. It grows in my yard as a volunteer, through the hedges, on the ground, and on my fence. I love it cause the Gulf Frits love it in the spring when my Incarnata has yet returned. Zebra Longwings will use it but not as much. I thought for a long time that it was Lutea, still haven't found pictures to compare the two but it does make great ground cover.