にせよEven if, Even though, No matter, Whether… or

Both mean whether…or…, but tone differs. にせよ・にしろ can be used in everyday speech or writing, as in 男にせよ女にせよ参加できる (Whether men or women, they can join). であれ〜であれ is more formal and academic, usually with nouns or な-adjectives, as in 貧乏であれ金持ちであれ人間は同じだ (Whether poor or rich, people are the same).

Both share an even if nuance. にせよ・にしろ asserts that the conclusion doesn’t change, as in 成功しないにせよ挑戦すべきだ (Even if you don’t succeed, you should try). たところで adds futility, as in 今さら謝ったところで遅い (Even if you apologize now, it’s too late).

Both can mean even if or no matter, but nuance differs. にせよ・にしろ stresses the speaker’s strong insistence, as in 学生にせよ社会人にせよ責任はある (Even if you’re a student or a worker, you have responsibility). であれ is more formal and written, as in 理由が何であれ許されない (No matter what the reason, it’s unforgivable).

にせよ・にしろ follows a single phrase to mean even if (A), as in 値段が高いにしろ欲しい (Even if it’s expensive, I want it). にしろ～にしろ lists two alternatives to mean whether (A) or (B), as in 行くにしろ行かないにしろ連絡して (Whether you go or not, contact me).

を問わず follows a noun (often a category) to show disregard for distinctions, as in 年齢を問わず (Regardless of age). にせよ・にしろ also implies that something holds true under different conditions, but it typically introduces a possibility that is then contrasted or disputed, as in 行くにせよ行かないにせよ (Whether you go or not…). を問わず highlights inclusiveness, while にせよ・にしろ highlights hypothetical contrasts.

Both show concession, but differ in nuance. にせよ・にしろ makes a strong assertion that something holds regardless, as in 結果が悪いにしろ挑戦するべきだ (Even if the result is bad, you should try). とはいえ concedes a point but softens with that said, as in 難しいとはいえ不可能ではない (While it may be difficult, it’s not impossible).

Both include しろ but are unrelated in usage. にせよ・にしろ means even if (A), as in 結果が悪いにせよ挑戦するべきだ (Even if the result is bad, you should try). 何しろ appears at the start of a sentence meaning anyway or after all, as in 何しろ忙しいので行けません (Anyway, I can’t go because I’m busy).

どうしても and にせよ・にしろ both express that something happens regardless of conditions, but differ in nuance and structure. どうしても comes at the start of a sentence to show persistence or inevitability, as in どうしても勝ちたい ('I want to win no matter what'). にせよ・にしろ presents hypothetical cases and emphasizes that the outcome won't change. For example, 雨にせよ雪にせよ中止しない ('We won't cancel whether it rains or snows').

にしても and にせよ・にしろ both mean 'even if' or 'regardless of', but にせよ・にしろ are stronger and more formal, often used in instruction or insistence, e.g. 行くにせよ行かないにせよ連絡してほしい ('Whether you go or not, I want you to let me know'). にしても is softer and more reflective, e.g. どこに行くにしても携帯は持っていったほうがいい ('Regardless of where you go, you should bring your cell phone').

Both show that something remains unchanged regardless of conditions. たとえ〜ても presents a contrasting hypothetical, e.g. たとえ雨に降られても気にしない 'Even if I get rained on, I won't care.' にせよ・にしろ is more concrete and neutral, often listing options, e.g. 行くにせよ行かないにせよ、連絡してほしい 'Whether you go or not, I want you to contact me.'

Both forms can express the meaning of 'even if'. ても is more versatile and commonly used in both everyday speech and writing, while にせよ・にしろ is more limited in usage, with にせよ sounding especially formal. Note that ても is often used in the pattern てもいい 'it's okay to (A)' to ask or give permission, whereas にせよ・にしろ is not used this way.