Problem 1
You don’t need to mark in when copy and pasting.
1. Just park your timeline indicator where you want the clips to go.
2. Alt + Drag (from the left) over the clips you want copied.
3. Ctrl+ C, (Standard Copy cmd), then Ctrl + V(Standard Paste cmd.)
It’s less than 5 steps.
Problem 2
At work we just mixed down a clip of black for 30 secs and saved in our Fx Bin.
We load it ( Already has 4 sec duration In and Out set)
Ctrl + A then splice in.
Problem 3 and 4
Read the manual about customizing your keyboard and what commands go where.
Under Settings — keyboard