Others could probably answer better, but my take anyway is that, with all the satipatthana-related suttas and parallels, it’s pretty likely there was an original satipattana sutta, but just probably a bit more slimmed down than the current Pali version with less body techniques (anatomical parts meditation seems common to all and corpse and elements meditations seem common enough in the sources too). It isn’t as if the extra further techniques are pulled out of thin air anyway; they are from elsewhere in the canon and there’s a certain logic to their inclusion (the first tetrad of breath meditation does make a lot of sense). I’m not sure it’s all that big a deal if some of these techniques are in or out (all seem perfectly good to me, included or not; maybe the original was simply a fairly terse satipatthana summary, maybe a bit too terse, which some of those later couldn’t resist fleshing out a bit).
Maybe there’s significance in the dhammas section being likely a fair bit slimmer as well (with just the seven enlightenment factors and probably hindrances also). However, the seven enlightenment factors are very prominent in the anapanasati sutta also (developing these seem to be a core purpose of anapanasati) so their prominence cannot be controversial here either. My take would the fact that the seven enlightenment factors are shared in all versions and the hindrances by most makes a good case that these are the most central initial dhammas to focus on. Maybe once a good level of samadhi has been cultivated (6th enlightenment factor), some of the other dhammas (aggregrates, sense-spheres, 4 noble truths) are more liable to be penetrated (on a satipatthana second round 
).
On solidity of the canonical texts, Buddhism is fortunate compared to many other religions. The Jains seem to have lost a lot of their early texts. Christians have to make a lot of inferences from rather a small amount of text (mostly the four gospels and Acts). The Buddhist EBTs are big and there’s a lot of shared overlap between the various parallels (so there’s a lot we can fairly confidently trace back to fairly early). On the downside, it would have been nice if there were some really early meditation and practice manuals (not really a written culture I suppose and the emphasis seems to have been on preserving the Buddha’s word rather than techniques of the time). But I guess we are still lucky to have some meditation manuals from some centuries afterwards.