It's also been posted several times that it isn't impossible for both men to have missed Jane in the seconds it took her to pass by if Gerry had his back to her - so wouldn't see her anyway - and if Jez was not looking in her direction in those seconds it took for her to pass them. They were also talking and listening to one another - not listening for extraneous noises.
Gerry did not need JT to give him an alibi - Jez - an independent witness was already his alibi.
Also if Gerry thought they could be witnessed by other people who could prove JT didn't pass by - then why ask her to lie in the first place?
IMO the three of them all told the truth - as each one of them remembered it. The fact that they have different recollections is quite normal.
It was a gamble to ask Tanner to lie but what choice did he have ? He knew the Smiths had seen him, and possibly would be able to identify him, so what better way to neutralise that sighting than by having a witness placing the abductor and Gerry at the same time. If Gerry wasn't the abductor Tanner saw then he couldn't be the person that the Smiths saw. Simple.
Of course this course of action carried two huge risks, either that Jane would crumble under questioning and admit the sighting was made up or that Gerry and Jez, and no Tanner, were seen talking by an independent witness. So Gerry simply put himself on the other side of the street with his back to where Tanner would have walked by and that way if it all went pearshaped he would come out of it untainted.
It was also of course a risk to place himself on the other side of the road when both Jez and Tanner disagreed with him but easier to explain away a misremembered location than seeing an individual who it is proved later wasn't there at all.
As to those few seconds it would have taken for Tanner to walk past , Tanner would have been potentially visible to either one of the talking men as soon as she left the tapas entrance to just before she turned the corner of the road which would have taken anywhere up to half a minute, so hardly the few seconds you suggest.