These need no basins. Several of the points you have raised could be explained by extensive, if not global oceans (Hematite distribution). Such a thickness of evaporites as seen in Meridianna would require a lot of water. On earth, the entire Mediterranean sea evaporated twice, and with several seas worth of additional water that flowed in before complete closure to produce salt beds about 500 feet thick. The Magnesium sulfate of Mars could form a 35.7 % solution at 20 C, but it is more likely that the original salinity was much less. Even at that, the 500 or more feet of evaporite type sediments in Meridianna would have needed at least 1500 feet of available water to have dissolved all this salt. I say "available" because both the salt and/or water could have been replenished, or recycled, and not present at any single time.

You criticise someone as to whether or not there is a basin in this eroded peneplain, yet your own favored theory requirea a basin. Sure Meridianna could have been a spring replenished Sabkha for the millions of years needed to get this thick accumulation of perfectly flat finely bedded sediments, but that is really a stretch. The least complex and most obvious scenario explaining the available evidence, planet wide, is global oceans (IMO). If it was a Sabkha, how come there are no mud cracks or rain imprints which are so common in that and related environments? Why would springs flow if there was no basin?