1.) Eurogamer was first to leak the hybrid form factor of the Switch while it was still known as NX so unless that was a wild guess they DO have sources. I've yet to see a single credible source point to A72, let alone A73. Nvidia seems to be still using A57s on Parker, their own Pascal Tegra SOC.
We have a Pascal Tegra with dual channel LPDDR4 and 2 Denver 2 cores, but it still has only 2SMs and A57 cores, not A73s. Perhaps Switch will have Denver 2?
Remember, NVidia and Nintendo started work on this SOC 2 years ago, so they're limited to tech that had to at least be in the planning phase then or the tech becomes an endlessly moving target.
2.) Switch UE4 defaults are level 2 for docked mode, level 1 for handheld mode. XBone, PS4 and PC are around level 3 default, so to put this into perspective, handheld mode has 40% the clocks according to Eurogamer, therefore, wouldn't UE4 level 2 be ~40% under UE4 level 3? That should tell you the power right there as a:
3.) Pascal @16nm FinFet with even 4 A73 cores and 3-4SMs would be ~65-90% the power of XBone, with the ARM CPU being clearly superior, and would not need lower UE4 settings.
Nintendo seems to be heavily prioritizing battery life and stability at the expense of sheer performance for Switch, but we'll see in January if that's truly the case.