Hmm... I don't have everything known to the dot, but I'll try to answer your questions.
Do note that I have no values to give you whatsoever. This is mainly talking from past threads I've read.
Also, I don't play Ranked. It's just not for me.
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This is dependant on the skill tier from which a player is to decay iirc. Lower tiers have a bit more leeway before the decay kicks in.
Decays system start from Tier 5 Bronze.
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Again, I think it's dependant on the tier the player is at upon starting the decay and where they are decaying to. For example, a Tier 10 decaying to Tier 9 will lose more Rank Points than a Tier 9 decaying to Tier 8 and so on.
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All the way to Tier 4 Gold. The decay stops there.
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As far as I'm aware, playing any Ranked Match will prevent both Tiers from decaying.
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Once you hit 6, you will get the skin, regardless whether your rank (in either 3v3 or 5v5) decays.
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Hope this helped a little bit.