Question 1) It'll be hard for us to visually determine the spring rate for those considering they are all different heights, overall diameters, and wire diameters. All of those dimensions are variables that determine the spring rate for any linear (non-progressive) spring. squeeze down on them and see which is softest (likely the small one) and install that. If you're installing a boost controller, I feel it's safer to keep the softest spring in the gate. You probably wont want to jump right out on 14+psi anyway. get used to 7psi and ensure the kit is holding up, all vacuum lines stay on with zip ties, engine is sound with no leaking, then turn the boost up with your controller.
Question 2) your wastegate theory is flawed so I'll clarify. Consider the hot side and cold side as separate systems that work at their own pressures and velocities. You need the intake pressure to be a certain value (10psi for example) an you'll have to bleed pressure from somewhere in order to achieve this. If you bleed from the intake (imagine an intake wastegate, not blow off valve) you will achieve your desired boost pressure but you'll continue to drive the exhaust pressure (turbine speed) higher and higher as the engine climbs in RPM and load, increasing back pressure, heat, power loss, and turbo wear. You'll also continually increase intake flow which could increase temps. We instead bleed pressure from the exhaust in order to relieve the back pressure, control heat generation, and not inefficiently overdrive the turbo. Now, the wastegate is our means of cracking exhaust pressure to stabilize turbine speed and boost pressure. The gate has a spring inside forcing the valve shut and it actually requires tremendous pressure to crack this valve. If you had no boost line running to the gate, it could easily see 25+psi at the manifold before the exhaust has built enough pressure to crack it. This seems counter-intuitive when the springs are rated at a given pressure (10lb spring in this example) but it does mean something... You plumb the boost reference line to the bottom of the gate (below the diaphragm) to aid in lifting the valve and relieving exhaust pressure. For this 10lb spring example, when the compressor side of the turbocharger produces 10psi (seen at the manifold), the pressure pushes the diaphragm up, opening the valve and stabilizing the turbocharger, so as not to produce MORE pressure. put a 15lb spring in the gate and it now requires 15psi from the compressor to lift that same valve so the manifold will then see 15psi.
Now adding the boost controller, as you mentioned, does bleed off some of the reference pressure to the gate diaphragm. the more you bleed, the more is required by the compressor to lift the gate, adding to the pressure seen at the manifold.
Sorry if that was convoluted beyond comprehension. I can put it more frankly if need be. lol