Investing in Yen vs the Dollar

So let's say I invest in the S&P 500 through a Japanese brokerage in yen. Then for example the yen tanks from 150 to the dollar to 250 to the dollar. However the index I used for the S&P 500 mutual fund increases by 40% over 3 years. Would the investment seem artificially larger than the 40% as a result? Of course, I know I would need to pay more in Japanese taxes but if I use NISA would that offset any losses from taxes?

Also as a side question, am I allowed to have multiple NISA accounts to offset taxes?

Thank you in advance!

by Icy-Highway-5980