にしても: a requirement that survives an accepted premise
Read a judgment that remains valid even when a situation is accepted, without turning an assumption into an established fact.
Accept a situation and still make a point
参加しないにしても、返事だけはしてください。 means that even if you are not participating, you should at least reply. It emphasizes that the minimum request for a reply remains regardless of participation.
Depending on the conversation, nonparticipation may be an established plan or an assumption that has not been settled. にしても alone does not establish that the decision not to attend is already final.
Connect plain forms and preserve negation
少し高いにしても、長く使える品を選びたいです。 means that even if it is a little expensive, the speaker wants to choose something that can be used for a long time. The price is accepted as a premise without changing that preference.
Positive and negative verb forms both connect directly. 行くにしても and 行かないにしても set different premises: even if someone goes, and even if someone does not. Do not overlook the negation while concentrating on にしても.
Shared meaning and focus with ても
参加しなくても、返事はしてください。 also asks for a reply even without participation. にしても can give the sense of accepting someone's choice or circumstances for the moment before continuing with a point that still matters.
Both expressions can show that the preceding condition does not cancel the following request. Rather than assuming one is always more polite or always hypothetical, identify what is accepted and which judgment remains unchanged.
What action does the speaker continue to request in 参加しないにしても、返事だけはしてください。?
Show the answer and why
A reply. Even if nonparticipation is accepted, the speaker asks for at least a reply. Do not turn it into a sentence insisting on participation itself.