Can I increase the length of LED RGB 5050 tape?
I'm not familiar what that product specifically, but the "amplifier" in this type of lighting setup generally only reads the input from the previous upstream light strip and provides power from its power source. Therefore, the amplifier draws no current from the upstream lights.

Instead of 2 power supplies, you could use one power supply larger enough to satisfy all of your power needs.



To answer your questions specifically:


Yes.
No. Power supplies are rated for the current that they CAN deliver. Using a power supply larger than needed is fine.
Again, not knowing these devices specifically, I am going to guess no based on my experience with other, similar products.
Generally no, provided you do not draw more power than the supply is rated for.