Spaniels are bred to retrieve, so carrying something is a natural instinct.
There's nothing wrong with giving a dog something to carry to meet this need, especially if it prevents an unwanted behaviour.
As far as the OP goes, it sounds to me like he's enjoying the game... he's got a ready made tug toy and is having a whale of a time. Lead chewing is generally a displacement behaviour (because they don't like the lead - normally young puppies), or it's something that has become rewarding for some reason.
I'd focus on an alternative behaviour... so train him to look at you, or hand target, and ask for and reward this BEFORE he starts mouthing the lead.
At 6 months, you are at a point where you are going to have to reinforce a lot of basic commands - keep everything fun and lighthearted and you'll get there!

If he is guarding things, though, it might be worth asking the advice of a (positive) trainer or a behaviourist, just for some pointers - one consultation now could give you all the tools you need to survive adolescence and prevent any problem behaviours becoming ingrained.