The present policy of acquiring more and more B738s is undisputable and I spotted that in the last three years they have acquired 6 x 17 year old B733s, which upholds your view that JET2 consider them a useful compliment for city routes and less popular destinations. Perhaps this trend to acquire ‘younger’ B733s to replace the older ones may well continue and could be the solution to the very old QC version. The question is whether the QC B733 is manufactured as such or whether it is a conversion.
The possibility of an early Airbus order, if the conditions are right, with a delivery in about three years’ time should not be discounted. Remember that JET2 has 8 fairly watertight bases and could easily designate any of these bases to be Airbus only, the same as has happened to Glasgow, which is B738 only. The aircraft rarely move between bases and neither do the pilots and other crews, mechanics. So long as the carrying capacities of the different types of aircraft in the fleet are the same/similar, then back up aircraft are always available, which is the most important aspect due to EU261.
The leasing of an A330 for next summer is still being mooted but as you say 2 or 3 B767s to replace the older B757s would be ideal if they finally decide to remain exclusively Boeing and would cover the New York shopping trips and open up other LH possibilities for JET2 Holidays. Time will tell and probably before the end of the year, as Norwegian will need to start distributing their 100 Airbus order between airlines and leasing companies, unless they themselves are going for a mixed fleet.
Not sure if I'm spotting correctly but the new registrations for JET2 aircraft could be interpreted as meaning, Jet 2 Holidays (JZH). There are now four such registrations, JZHA, JZHB, JZHC and JZHD.