Not true. USD is backed up by a State entity; bitcoin doesn't need one. An alternative USD needs an alternative guarantee of its value, an alternative bitcoin is as safe as the original one.
Not quite, since the "backing" of Bitcoin is in it's proof of work, an alternate won't have as much hashpower backing the work, so it won't have as much guarantee.
Every now and then I like to look at the chart at [0] to show just how important this is.
The most prominent "fork" of bitcoin (bitcoin cash) is still only about 45 days away from having their entire history since the fork rewritten using just 25% of the available hashpower.
In a little over a month, they could re-mine every single block since the fork changing which transactions are included. And that's the most popular fork, rewriting the entire chain since the fork. If you want to rewrite just the last 50 or so blocks? You'd need something like a few days and a few percent of the overall hash power to do it.
Are you in possession of like 1% of hashpower? Then it's more than possible for you to send something like 100 BCH to an exchange, wait for the 10 confirmations, then go to work rewriting the chain from that point to "current" without your transaction.