I doubt a celebrity or influential person will endorse Monero. Especially when it's labeled as a "coin for criminals" by mainstream governments. Would anyone want their reputation tainted by supporting a controversial cryptocurrency?
Why not? There are several celebrities with a rebellious image. Take (punk) rock stars, gangsta rappers, certain actors, "business punks" ... Their image actually could benefit from endorsing Monero.
There also could be celebrities for example supporting opposition activity in dictatorships like Russia, and those people could explain the public why Monero is good for these activities as they make it more unlikely to be detected and prosecuted by their respective authorities.
Perhaps the XMR community should reach out to this kind of personalities
Merchant adoption will continue to decline out of fear from government prosecution.
As long as there is no ban on Monero, there is no reason for a merchant to cease XMR support due to "fear from government prosecution".
If XMR merchant adoption is currently decreasing (I don't know numbers that could confirm that), then it's probably related by their favourite crypto exchange having delisted Monero. This is of course a real danger. So it depends on the Monero community if their decentralized exchange tools, like Haveno, grow to maturity fast enough to replace the centralized options.
About Monero at $1: I see no reason why it should not be used as currency in this hypothetical case (as I wrote before, XMR is currently a well performing crypto, so this is "very" hypothetical
). However, a fall to $1 should have a reason, and it is likely that a dramatic loss of adoption should have occurred before this mega-crash happened (99.4% compared to current price).