grammar - ですら・でも・さえ・だろう（と／が）。 Are there any differences?

I am just wondering if anyone can explain the differences between these in terms of nuances.

When I say nuances, can you give examples of whether one is for more polite situations, or whether one is more akin to regardless of whether X, Y or even if X, Y, or anything like that.

I just can't see myself using anything other than でも if I am stuck believing they are all the same.

Answer

Overview

These words have their own meanings, only being alike when translated into English. Those other than だろう(と/が) don't convey any politeness or formality by themselves.

The "regardless of whether" sense is shared by: でも and だろうが.
(but the best choice is であるかにかかわらず, I think.)
The "even if" could be shared by: でも, すら and さえ.

だろう(と/が) vs. the others

だろう(と/が) is the odd one out here, which has little chance to be used in similar situations to the others. It could be translated into "no matter ..." or "whatever ...", always connoting subjective emphasis, usually associated with decision, irritation or even disdain according to the context. It could be formal, but no way polite (neutral ~ offending).

You can duplicate two or more units: A だろう(と/が) B だろう(と/が) "no matter A or B"

The grammatical breakdown is だろう "will/would be" + と "when, whereas" or が "though". だろうと is literary, while だろうが is colloquial. Note that だろうと shares the form with other expression, namely "(say) that ... would ...".

でも vs. さえ/すら

でも is basically an expression for enumerating adequate instances "as like ...", while さえ and すら are for convincing by one extreme example "even ...". However, a single でも could be used as substitution to さえ/すら, since the only example tends to be used as the most powerful exemplification.

こんな問題、子供(でも/でさえ/ですら)解ける。 Even a child could solve the problem.

難しい問題でも考えているのだろうか。 Is he/she thinking on a difficult problem or something?
× 難しい問題(でさえ/ですら)

As you can see, でも can be duplicated: A でも B でも "either A or B", but さえ/すら not.

でも originates from the combination で "in (the case of)" + も "even, too", thus inevitably shares the form with many other irrelevant expressions, due to で's ambiguity. Especially でもある is も incorporated in である (=だ), and usually has nothing with the expression.

さえ vs. すら

As said above, さえ and すら are used to focus on the most significant example. There are few reliable sources which could explain the difference between them exhaustively. In my opinion, their usage is somewhat merging, and highly interchangeable, except for one point observable: A さえ B usually means "even [A do B]", while A すら B is "[even A] do B".

宿題さえなければよかった。 If only there weren't homework.
宿題すらなければよかった。I wish if there weren't even homework.

○ 愛さえあれば何もいらない。 I need nothing as long as I have love.
? 愛すらあれば何もいらない。 ?? I need nothing if I have even love.

Grammatically, when the bare さえ/すら are used with transitive verbs, the words before tend to be interpreted as the objects. Use でさえ/ですら for the subjects.

君さえ止められない人間を見たことがない。I've never seen someone who even couldn't stop you.
君でさえ止められない人間を見たことがない。I've never seen someone who even you couldn't stop.