You have answered some of your own questions, before I got to this, but I will add some additional thoughts.
First, as Seachem told you (and me some time back), it is best in their opinion to wait 24 hours after using Prime before adding Flourish, and their response explains why. Some others think this is unnecessary, but I wait a day after the water change to add my first of two weekly doses of Flourish. No harm is done by waiting. And I would mention that this applies to any water conditioner that detoxifies heavy metals, and most do. I for instance do not use Prime becaue I don't need or want all that it does with respect to ammonia and nitrite. Now to your other points.
Can plants still use de-toxifyed metals? Yes/No or to what degree?
As Seachem explained it, no, so I will leave that.
Is this the reason Seachem manufacturers other supplements like iron and trace elements to name two?
No. Plants require 17 nutrients, and in certain proportion to each other. This has to be balanced by sufficient light (intensity, also duration). This complete balance can be at different levels, and light is the catalyst. Those of us with a low-tech or "natural" method planted tank use moderate light, and a once or twice weekly dose of Flourish Comprehensive is all we need. Flourish has all nutrients except cabon,oxygen and hydrogen--all of which occur naturally in the aquarium--so it provides everything and in proportion.
Going up a level, when you start increasing the light intensity you need to increase everything else to balance. And that is where the other products come into play. In other words, they are intended more for higer-tech systems.
How can Flourish be a complete fertilizer if a fishkeeper uses Prime that affects the heavy metals that are listed in the Flourish?
Why does their Trace elements have more ingrediants then those listed in Flourish?
I think I covered this above.
So this may be a reason why one needs to add a higher dosage beyond the recommendation or to use their Iron supplement and Trace elements to compensate.
No, as explained above. And adding too much of some nutrients can cause real problems. Some are toxic (like the metals iron, copper, etc). Some will cause plants to shut down assimilation of other nutrients, creating an artificial deficiency. This is why it is important to add everything in proportion. And, in some situations, certain nutrients may be lacking. For example, I have very soft water, so calcium and magnesium in partcular are almost lacking and there is insufficient of these in products like Flourish to make up the shortfall. I use Equilibrium for this, and I note you mentioned it too. BTW, add Equilibrium right after the water change. It is only hard minerals and these are not affectd by Prime or similar conditioners.