Single minded describes a person with an objective they pursue in a determined manner.

That’s a load off your mind is an expression used when a cause for concern has been resolved satisfactorily, for example.

Jumping to conclusions is something people frequently do, when they do not have enough information.  It usually describes an incorrect guess.  I seem to remember it was used in an amusing way in The Phantom Tollbooth.

Your guess is as good as mine is a way of saying I really don’t know.

I don’t rightly know means I’m not sure.

How did you guess? A question sometimes asked when someone has just stated the obvious.

If it doesn’t do any good, it won’t do any harm.
This could apply to a home remedy or an attempt to put something right.

They didn’t come to any harm might be said after learning of some risky behaviour, which did not lead to disaster.

To cushion a blow is to reduce the impact of some bad news by breaking it gently. 

Shut your trap!  This is a very impolite way of telling someone to stop talking.

It could go either way means the outcome is not certain.  It should only be used where there are only two possible results, such as pass or fail, win or lose.

It’s in the balance is a reference to weighing scales.  A balance has been used to compare weights since early times.  Nowadays we have bodies such as the Trading Standards.  The Bible urges people to be honest in their dealings and to use correct weights and measures.  Proverbs 16:11

That would open the floodgates…

…by setting a precedent.

Work it out for yourself!  The speaker is not going to explain.  “You should be able to work it out for yourself” might be gentle encouragement.  The Bible tells us

Work out your own salvation”.  This is not problem solving.  It is being a disciple.  Philippians 2:12-13