After having examined the data you've found and posted, I have a few thoughts - none of which are conclusive.
The photo you reproduced earlier in the forum thread looked like it dated from the 1970's (based on a car in the background and the hairstyle of the resident), the designation reports are dated from the early 1980's. Both describe a "mound" that is large in diameter and only 3-4 feet high. (This would mean at its highest - probably measured from street level.) Having seen a number of barely discernible mounds in the midwest with more impressive numbers, I'd guess that sometime after your childhood memories of a taller, more circular affair, the mound was substantially "smoothed" off by the road rerouting and some obvious infrastructure changes that go right through the long axis of the green space.
The google street map view shows a below ground electrical line, a below ground run-off sewer, and a water line. (You can see the streetlamp, electrical boxes, storm grate, and faucet.) Your old black and white photo shows mature trees, the modern view shows none - except the half dead one at the east end of the greenspace, across the alley - a guess that other trees have been removed - another possible damage point for the mound if they were bulldozed.
I'd take a guess that only a portion of the base of the mound is still in situ.
The cave - well certainly no sign of it. Was it right by the mound? Or between the mound and the river? The El Portal village website has a map with the building dates of homes superimposed. Three of the homes south of the greenspace were build after 1970; several show signs of recent additions. It's entirely possible that the root cellar/cave was built over, or simply filled in to prevent children, tramps, or under "undesirables" from messing about in it.
Contacting some of the names on the El Portal website, associated with the "archives" might get you some more definitive answers.
Something is not right. This message is just to keep things from messing up down the road