So every time I see the amoeba given credit for doing amazing things I feel that much less guilty.

To put it more plainly, I don’t think it’s intent that matters. I think it’s a question of use and timing. In the case of the slime mold, they refrain from eating and take the bacteria with them on a long journey. When they arrive, they plant and wait for their preferred food source to multiply, and then feed on it relatively soon after arrival. Without the seed, there may have been little or nothing to eat. Thus the amoeba that brings the “seed” benefits directly and relatively soon from its transport of foodstock. Personally, I’d call that, but not what the birds do, farming. Good thought experiment, though!

As for the origins of multicellular life, I think slime molds may tell us a lot, but never the whole story. They started out as amoebas, and amoebas have their own peculiar characteristics and abilities. Plants, animals, fungi, and the other major multicellular groups probably started out as very different single-celled organisms, and early aggregations of those kind of cells may have had very different properties than slime molds do. It’s also worth mentioning that those early attempts at multicellularity took place in an aqueous environment — the ocean. I don’t know what sort of difference that might make, but probably some because marine organisms don’t have to fight gravity the way those of us bound to land must. On the other hand, it is possible this sort of in-between phase may have happened several other times in Earth’s past in spite of these differences — but without one hell of a lucky fossil, we’ll probably never know!

But I wonder, is farming defined by its intentionality? If birds propagate mistletoe berries unwittingly, fertilizing them with their feces and “planting” them on the tree branches they’re bound to frequent again, we don’t call it farming, even though the effect is similar. There must be a certain amount of intentional care paid to the berries’ development, right? But if intent is key, can a completely witless organism like a slime mold be said to really “farm,” when it seems to be merely, as you put it, packing a lunch?