So the formula is pretty simple. The 'points' needed to guarantee raising a skill by one level is equal to the current skill level multiplied by a constant (which varies depending on the rarity)
R is 5, so it's skill level * 5. SR is 10. SSR is 20.

How many 'points' a particular piece of fodder is worth works similarly, but the constants are different this time around.
R is 10, so it's skill level * 10. R grail is 20. SR is 35. SR grail is 50. SSR is 350. SSR grail is 400.

(as you can guess, the probability of success is points_from_fodder divided by points_for_100%_raise, with a quotient above 1 being meaningless)

The shortcut way of thinking about this is that an SR needs the equivalent of 1 R (at skill lvl 1 each) per level, while an SSR would need 2 per level. But of course, in practice, you won't be using literally 1 R per level for an SR or 2 R's per level for an SSR, at some given skill level.

As for the 2nd question, whether to focus on leveling one skill or spread the leveling out depends on how you want the power/investment curve to look. The absolute gain from 1 skill level is constant, regardless of what level it is. So going from skill level 1 to 2 gives as much to you as going from 19 to 20. But the investment/fodder needed are drastically different as the formula above indicates. If you opt to raise a bunch of a weapons by a few levels, the gains are frontloaded. It's a quick boost, but the gain/investment rate sinks so you picked up all the easy gains. The graph would look like logarithmic growth perhaps.
If you focus on one weapon at a time, then the gain/investment curve constantly swings up and down. I'd say that given X investment, you've gained less this way compared to evenly leveling. Emotionally, you'd probably be going through a constant cycle of quick gains and slow gains, as opposed to 'after getting all of the easy pickings, the future just gets slower, and slower, and slower'.

Third question, excessive SR weapons are usually reserved as fodder for SSRs. The assumption is that in the long run, you're ideally shooting for weapon grids mostly/entirely composed of SSR. Ergo, SR weapons are presumed to be placeholders that you only invest in as necessary, while SSRs are what you want to eventually max out.