Which is one the best and right one:
Finally, I will be at both focal points' disposals should they need my support.
Finally, I will be at both focal points' disposals if they need my support.
And is using "Finally" here correct?
They're both correct.
If states that x happens, therefore y will happen because of x. This works here.
Should here is what's called an expression of subjunctiveness. The subjunctive refers to things that don't deal with objective facts. It implies wishfulness, possibility, etc. Your second example is called subjunctive with inversion and it is absolutely fine in English.
You can see that here in the example, should you feel hungry.
And the word finally is fine here, if this line is part of a list and this is the final item of that list. In any other context, I'd say it would be more awkward, but I'd have to see that context to know for sure.
Both the sentences are correct grammatically; there's no difference in meaning. The only difference is that the former sentence is formal
The sentences are expressive of conditional 1. According to Cambridge English Grammar Today, in formal English, you can use should + subject + verb instead of 'if'. For example:
Should you cancel your order, contact our customer services department. In formal English, you can also use if with should as follows:
If you should cancel your order, contact our customer services department.
So to be more formal, you can use the first sentence presented by the OP.