Hello! I am also very new splinterlands, however in my experience I have a couple of things to note.
1.) I would not say that there is a combat triangle per say. However, certain tricks can definitely help you win battles. For example, when going against an earth splinter, the void ability (in unicorn mustang and bone golem) can be very useful. Especially in starter decks where earth splinters primarily use magic attacks
2.) I have been playing with no money other than the spellbook. Progress can be slow and frustrating but you can still have fun and play the game.
3.) I do not know as I don't rent cards.
I hope this helps!
RE: Need Idiots Guide To Splinterlands