There's some money involved if you run a lightning node, but this requires owning bitcoin, so I doubt that's what OP is asking about and what he's able to do at the moment.
How much BTC is needed to run a lightning node? Is that BTC tied up with the node? Is it put at a risk?
Depends. The lowest amount to start a channel is the transaction fee + amount of coins you want to put in the channel. The less you put there the less transactions you will process.
Nodes are chosen based on capacity and they have their own rank. If you want your node to have high rank you need a lot of capacity. I don't run my own so I'm only telling you what I know about it, not from experience.
I saw a few people share their experience and usually if they had between 1 and 2 bitcoin locked in the network they were able to earn $10-20 a month from it.
BTC is tied to the node but it's not at risk.
And on that risk note, answers in this thread indicate that my transactions are more secure if I have my own full node.
Yes. You can route your transactions through your own node.
Looking in the other direction, my transactions are at more risk when I do not have my own node. Just how much more risk is there?
I'd say very little. Node proponents tell you to run it because it contributes to the network. The more nodes the more robust the network becomes and if you have a lot of bitcoin you want the network to work well. For someone with 10m dollars in bitcoin, spending $500 on a plug and play bitcoin node is dust.
If I am running a wallet such as Trezor, and I send or receive some BTC, how much risk am I exposed to?
If your system is not infected and you don't make any mistakes like send it to a wrong address or set the fee too high, you're not at risk. I've sent hundreds of transactions in the last 6 or 7 years and none of them was blocked, hijacked or lost.
I suspect that if I am using Coinbase, and put in a buy or sell for BTC, the only risk is that the dollar cost of BTC changes while the transaction is being consummated. Is there another risk I am not aware of?
Yes, but if you're paying for something and that transaction is sent, it's over. Payment processors send information to the store and it's the same with ATMs. If you exchanged bitcoin at 30000 dollars and by the time it confirmed the price was 29500, they won't cancel it or send the money back to you.