Inadvertent eDNA research

No actually. If you consider the plants to be Archaeplastida (glaucophytes, red algae, and Viridiplantae) or Viridiplantae (the green algae including Embryophyta) then the common plant ancestor is unicellular (greens and reds evolved multicellularity independently). If you consider the plants to just be Embryophyta (the land plants) then they already had highly specialized cells and looked plant-like before they split off from the rest of the green algae.

I’m not sure if the fungal common ancestor is believed to have been unicellular or multicellular but if it was multicellular then it would’ve been filamentous like modern multicellular fungi, rather than a sheet of cells

Fun fact: Animal embryos can be disassociated by depriving them of calcium (E-cadherin, the molecule that holds the cells together, needs to calcium to work) and then can be allowed to reassociate by adding back calcium. If you do this in early enough stages then the embryo will function and develop normally once reaggregated, despite all the cells being jumbled up

Early animals were likely very similar to Trichoplax, but they weren’t Trichoplax. Trichoplax adherins is a modern species with just as many millions of years of evolution between it and the first animal as between us and the first animal. Just bugs me when people end up implying that orthogenisis is real

I think you misread wikipedia when it talks about its endosymbioses. Whole bacteria are found within an organlle (the endoplasmic reticulum) of Trichoplaxs.

That being said what you described does happen in a number of organisms (including ‘complex’ ones like nudibranchs): they steal the chloroplasts from the algae they eat in a process called kleptoplasty. Seeing as mitochondria and chloroplasts originated as bacterial endosymbionts that were then heavily integrated into their hosts, calling kleptoplasty a form of symbiosis isn’t that unusual.