Q1: What does this card's (Rule) mean, exactly?

This is a special rule for this card that always treats it as if it also has the Aquatic type trait.

Q2: When I use this card's End of Your Turn effect to return this card to the bottom of the deck, can I activate this card's <Decode (Blue Lv.5)> and then return 1 of my opponent's level 6 or lower Digimon to the bottom of the deck?

Yes, you can activate <Decode (Blue Lv.5)> and return a Digimon to the bottom of the deck.

Q3: I use this card's End of Your Turn effect to return this card itself to the bottom of the deck, I use this card's <Decode (Blue Lv.5)> to play a card with an On Play effect, then I attempt to return my opponent's EX6-054 Lucemon: Chaos Mode to the bottom of the deck, and EX6-054 Lucemon: Chaos Mode's All Turns effect plays a card with an On Play effect. At such times, do the On Play effects on cards played by these effects trigger simultaneously?

Yes, they trigger simultaneously.

Q4: If I activate this card's End of Your Turn effect, return this card itself to the bottom of the deck, and use this card's <Decode (Blue Lv.5)> to play BT19-025 MetalGreymon, at what timing is the DigiXros performed?

You perform the DigiXros at the timing when the card is played. You perform the DigiXros when this card's <Decode (Blue Lv.5)> is used to play BT19-025 MetalGreymon, then you choose an opponent's Digimon to return to the bottom of the deck for this card's End of Your Turn effect.