Why did farmers, among others, prefer "free silver" over the gold standard?

Because free silver meant a monetary expansion, basically, it increases the amount of money in circulation because instead of only minting coins off of gold, it mints coins from both gold and silver. What this does is it increases the rate of inflation (because there is more money in the system), which decreases the value of debt, which farmers usually owe to banks or merchants. Therefore the net effect of bimetallism makes farmers better off since it reduces the real value of their debt. If this is unclear, please let me know because I might not be explaining it well.

For that matter, when people talk about reviving the gold standard today, are they making a historic argument -- it worked then it'll work now -- and is there any truth in that? [To the extent that this question can be answered within the subreddit's rules.]

I basically can't explain it in detail without going into a lot of economics, but the simple answer is that the gold standard is not very compatible with a modern economy and it was made obvious in the 1930s and again in the 70s. If you want a more in-depths explanation, I'd be glad to give it.

I'll take the question about why the gold standard facilitated international trade. (Similar principles also apply to other fixed currency regimes, like a dollar peg.)

The main reason is that it made currencies relatively stable.

Say I want to buy wheat from the US and sell it in the UK. I need to change pounds into dollars to buy that wheat. I then earn pounds selling the wheat. Hopefully I earn enough to buy more dollars, buy more wheat, and make a sustainable business.

But what happens if the value of the exchange rate between the two currencies fluctuates? If the pound goes up, I'm in luck, because I can buy more wheat for my pounds. If the pound goes down, though, I can buy less wheat. This fluctuation could even occur between me changing my money and buying the wheat. This makes international trade riskier.

Things are worse with investments because of the longer timescales. Let's say you've lent me dollars to fund my import business. To pay you back I have to buy dollars. If the value of the pound falls, dollars become more expensive for me to buy, making my debt harder to service. How do either of us know whether the pound will rise or fall over the lifetime of the debt?

Under the gold standard, things are more predictable. I know pounds are worth a fixed amount of gold, and I know dollars are worth a fixed amount of gold. True, a currency might occasionally revalue or devalue, but that's an exceptional event.

This leads into one of the problems with the gold standard. Let's say lots of people want to buy wheat from the US, and people in the US don't want to buy anything from them (eg. the US has put up high tariff barriers for imports). There's a trade imbalance there. Lots of people want dollars but Americans don't want other currencies. Normally if lots of people want something the price goes up. So international trade will reach an equilibrium, as dollars and thus US imports become more expensive.

But under the gold standard this can't happen! The value of those currencies are fixed against gold, and thus against each other. The central banks are forced to manipulate demand for their currencies, usually by buying or selling gold. So if nobody wants pounds, the Bank of England sells gold, buying back pounds. This works in the short term and against modest problems, but if you have a major or long-term trade imbalance, it becomes very difficult to manage (not least because a country can potentially exhaust their gold reserves), and has all sorts of knock-on effects. Edit: not sure I got this bit correct.