No; Grin doesn't require transacting parties to both be online. The necessary back and forth can be done by email, by direct message, by carrier pigeon, or what have you.
What do you mean by "Grin’s privacy depends on network activity" ?
Grin's hiding of amount and address doesn't depend on that.
Only the use of CoinSwap to hide the tx graph would depend on a sufficient amount of daily CoinSwap txs.
While Monero's privacy is a little better, it pays a large price for it: having a 20x faster growing chain, and the need to remember randomly accessible data from every output ever spent.
What do you mean by "Grin’s privacy depends on network activity" ? Grin's hiding of amount and address doesn't depend on that. Only the use of CoinSwap to hide the tx graph would depend on a sufficient amount of daily CoinSwap txs.
While Monero's privacy is a little better, it pays a large price for it: having a 20x faster growing chain, and the need to remember randomly accessible data from every output ever spent.