The idea of sitting next to something was trying the adapt the retrieve for a situation where the dog can't pick up due to the muzzle. The OP has struggled with this for some time, and there are genuine health issues being caused.
Ideally, yes, distraction and training would be the way to go, but if a behaviour is seriously ingrained, preventing it while working on the training is no bad thing.
We used to regularly walk with a Weim who was muzzled for about 6 months are her poo-eating habit kept landing her at the vets (with serious tummy issues). During the muzzled months, the owner was able to improve her recall and train a leave it - now the dog may occassionally have a nibble, but isn't poo-obsessed and is happily walked off-lead with no muzzle.
When training "leave it" and recall we are always advised to start with low value items, or low levels of distraction, and build up - scavenging has become a high-level distraction so would seriously impact on the ability to train a recall. A long-line wouldn't be sufficient in this case, as the dog could easily pick up and eat whatever while it was being reeled back in. A muzzle allows off-lead exercise to continue while training is in progress.
click the clicker to bring him back...
You should not click the clicker to get a dog to come. It is a marker of them doing the correct behaviour, not a recall command or a way of attracting your dog's attention.... I've lost count of the number of times I've had to say this to you. If you don't want to use to clicker correctly, that's up to you, but do be careful giving advice to others about it, as you could confuse the issue.
