What's the best option to repair a long asphalt driveway with heavily cracked portions?
Not sure about GemSeal or similiar stuff, but if You live in hard climate area, won't last for long, to be sure.
That will crack in next year or so.
Possible, but depends on how long these cracks are and what kind of these are (what forces cause this problem). I wouldn't recomend that.
Good idea, especially in harsh climate, where ground freezes in winter, and - if underbed of Your current layer is rather badly done. It may be done with other material than asphalt. There are some materials like brick paving. You could do this Yourself probably. Don't forget to make good substructure. 


If You are short on money, try 1.(not sure) or 3.(You will need some devices and helping hands).  If You got some savings and want to to this really nice and good-looking, choose 4. (which I strongly prefer).