A simulacrum, even true polymporphed, gets destroyed upon subsequent casting of simulacrum
tl;dr True polymorph changes a targets form and stat block. The property of being a simulacrum is not inherent in either of these, so true polymorph does not insulate prevent the simulacrum from being destroyed upon a subsequent casting.
The form of the simulacrum ...
No, your Simulacrum is even when true polymorphed just affected by a spell that can be negated by other means, it is still your simulacrum and would be destroyed. The simulacrums [I´m a Simulacrum] property is not a game statistic that can be changed.
My view is that it is functionally a Polymorph into a beast, so the imp loses the ability to speak. All part of the tradeoff - you get to appear as a harmless raven (or something), fly around and avoid suspicion, but if you want to talk you need to change back.
You do not get equipment always carried by the creature. As you pointed out in the question, polymorph is explicit about what it grants, and equipment is not included. In fact, the only interaction polymorph has with equipment is to absorb it for the duration if the caster is using equipment that the new form is incapable of using, too.
It is an insta-kill that requires a 9th-level slot and the enemy to fail a WIS-based save
PWK states that
If the creature you choose has 100 hit points or fewer, it dies.
And Polymorph states that
The transformation lasts for the Duration, or until the target drops to 0 Hit Points or dies.
Given that, from the SRD,
A creature that has died can't ...
There are no rules guidelines for this situation
Let's get the obvious out of the way. Equipment can be merged into the Wild Shaping druid, regardless if they were formerly creatures, so far so good. True Polymorph states that when it ends the target "returns to its normal form". However this is general advice, it is not normally assumed that a character ...
Yes, you can true polymorph yourself into yourself, as you are a kind of creature. As for people who use other completely different descriptions from completely different spells to reason that a similar word usage they all share makes the word use of the other completely different spell in question the same as said completely different spells, you are ...
Truesight sees both
Many animals can see into the ultraviolet. What does that look like? I have no idea because humans can’t see it, and we have no conceptual touchstone to describe what color ultraviolet is.
Similarly, I don’t know what it looks like to see the shape changed creature in both it’s changed and unchanged forms. But someone with truesight ...