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:
(but the best choice is
であるかにかかわらず, I think.)
The "even if" could be shared by:
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
だろうと is literary, while
だろうが is colloquial.
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
すら are for convincing by one extreme example "even ...".
However, a single
でも could be used as substitution to
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?
As you can see,
でも can be duplicated:
A でも B でも "either A or B", but
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
も incorporated in
である (=だ), and usually has nothing with the expression.
さえ vs. すら
As said above,
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
すら are used with transitive verbs, the words before tend to be interpreted as the objects. Use
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.