Here is a good breakdown of why people expect lower than 5ghz.
Everything you posted is pure speculation, though we do have actual factual evidence and that is Radeon 7 and MI60. Its essentially tweaked Vega on 7nm. What it does on 7nm is being able to provide 35% more performance at the same power consumption, with less CU's compared to Vega 64.
Radeon 7 is essentially a cut down MI60 running at 50MHz less, but MI60 runs at 1800MHz or 264MHz more than Vega 64. Again this is the same architecture with minor tweaks, no major redesign, nothing of the sort. Its a slightly tweaked Vega 64 on 7nm process essentially.
And again why is Nvidia able to run GPU's at 2100MHz, why is Intel able to run CPU's at 5GHz and AMD can't? Why can't they have improved their technology, improved their tools, with TSMC improved the 7nm process from the MI60 days, and we had 7nm sampled in Q2 2018, so its been a full year for the process to mature, it's going to be a year and a quarter when Ryzen and Navi hit the market.
TSMC recently announced their 6nm process node and they claim it's up to 10% faster than 7nm or 15% more efficient. So to me it's highly likely that AMD are using an advanced 7nm process, 7+nm if you will for their next Ryzen and Navi chips.
Again I don't see how it's unbelievable that they've been able to increase clock by only 600MHz on 7nm as opposed to 12nm, even if it means increasing the power consumption somewhat. It doesn't really matter if AMD's final power consumption is say 20W more than the leaked documents, it won't matter, as Intel's 8core chips consume much more power already, if AMD releases 16 cores at 150W that would still devastate Intel and blow them out of the water. Their perf per power per chip would be out of this world.
Again to me it seems very likely that on at least a one year mature 7nm process, on an upgraded Ryzen architecture they've been able to increase clocks by 600MHz.