It sounds as though the deals offered vary and depend on your qualifications. Historically Jet2 have not given employment contracts to anyone requiring the type rating until the type is on your licence. Whatever they offer you until then may change until you have the type rating and receive the contract to sign.
As for changing you from part-time (70% or whatever) to full-time 100% at the end of the summer period, I believe those currently in this situation are not being given any back pay. So basically Jet2 have had some cheap formally part-time now full-time pilots this last summer - no doubt something they may do again.