I challange anyone here to find a situation and and story that matches mine. None of you have any idea who I am, or who I take photos for.
I am attending these events as a guest who takes pictures. They have more chance of getting the Real Elvis to perform the service than they do suing me....
It's great that you are doing something you love and it's great you are helping people. I do think you've slightly taken the comments the wrong way insomuch that you are not considering there 'may' be another side to what you are giving.
You are of course correct that it is incredibly unlikely that you would ever be sued, but I think you are incorrect in believing they have more chance of Elvis turning up than successfully suing you. The simple fact is as a guest or not if you have talked with them before hand and agreed they will get some pictures you are at risk if things went wrong.
I don't doubt your generosity or motives, I'm starting a 'free service' myself making portraits for families with disabled and terminally ill children, hey will get the session and prints for free without some catch to buy more prints at the end, but trust me every single one will also be signing an agreement with me before hand. The reality is in my case I could most likely do the sitting again if I go went wrong or they could go elsewhere and pay, with a wedding day if 'things went wrong' they can't easily do it all again.
I'm pretty sure that's all anyone wanted you to be aware of. Sure there will be people out there who don't want you to do it for free, I don't believe anyone in here cares if you charge or not and they certainly have no need to be jealous or bitter. The point was you didn't see the potential issue of things going wrong. All you need to do is sign an agreement with them, and then they know exactly where they stand. If you are doing it purely as a guest with a few pics then it's a pointless discussion, however if you are the main photographer (guest or not) and have the job of making pictures from their big day then you need to protect yourself.
Keeping your current camera as an emergency spare is a good idea, most wedding photographers will use two cameras during the day for speed and convenience and the second is also a backup, but many of them would have a third on the car. Cameras pack up, lenses do too, I was reading something a week or so ago from a guy who was at a wedding and two lenses packed up within half an hour of each other. What are the chances? No idea, but it happened. He had other lenses, not what he wanted but he was able to get by.
There is a really good chance that if the camera packed up you could put your hand up and say 'sorry guys, I can't carry on as my camera has died' and everyone would say it's fine and not to worry. But what if they didn't? If it got to court would you win? Maybe Elvis could be your lawyer
My first few posts here were spent arguing too, but it's not a bad place, people are just passionate and have seen so much of the same stuff before, too many have come before you doing weddings cheap or free and getting it wrong, heck some charge a fortune and mess it up, don't take it so personally. People were genuinely trying to caution you that things can go wrong, even if you believe you are bullet proof I wouldn't want to test my verbal contract in a court of law facing an angry bride and her p***ed off parents. You are a clever guy, surely you realise the world isn't perfect and people sue anyone for anything. As an engineer you understand the importance of finite details, so make sure you have every angle checked and you won't have any issues.
Best of luck with your adventure.
Oh, and the 6D would be a great choice