Beatrice, Lady of the Eternal is not a Burning Abyss monster and your Malebranche cant coexist with her
Finally, the ultimate isn't even good all the timeif you are winning and have a decent board, you might want to keep your own permanents rather than take your opponent's, especially since Aminatou, the Fateshifter is more about synergy than raw power, so your cards will likely be more valuable to your deck than your opponent's cards
There's no limit to the number of trainer cards you can have in your deck, but around 20 is usually a good amount
With a current market price sitting at $132, this card has seen some recent dips in the 7-day and 30-day trends (-2.21% and -11.15% respectively), but don't let that fool you
Then you should look into prices of the last sold card at the higher grades for the selected cards