1) It was an unfortunate fluke that Gerry was spotted by the Smiths. As he said himself he had not seen anyone while checking any other evening. Any plan to negate the sighting was constructed between the sighting and the arrival of the police.
2) Gerry's alibi was that he was in the same place as Tanner's sighting ( who of course he relentlessly pushed as being the abductor). Link, as the McCanns attempted to do, the Tanner and Smith sighting and Gerry couldn't be the man seen by the Smiths.
3) Until we know exactly what happened that night we can't know how much Tanner knew before she gave details of her 'sighting' or why she did it.
4) We have no idea whether Tanner has 'cracked'.
1) why would you describe the Smithman sighting as "a fluke"? If you're walking through a holiday resort whilst all the bars and restaurants are still open then it can hardly be called a fluke if you pass at least one person in the street, more like a cast-iron certainty. I'd be interested in why you think Gerry asked Jane Tanner to lie for him, and not someone he was closer to - any ideas? What opportunity did the two of them have to be alone together that evening in order to hatch the cunning plan?
2) Gerry's relentless pushing of his alibi is somewhat negated by his claim not to have seen JT as she walked by, thus throwing into complete doubt the veracity of her alibi in the first place. Hardly sensible is it?
3) Meh
4) So you think it's possible JT has "cracked"? I would have thought that would have resulted in an arrest of Gerry McCann by now wouldn't you? Why do you think this hasn't happened? Are they tick-tocking and catching the monkey softly as usual?
