Pre TOPS '00' locos tended to gain the last number in the new series eg D9000 became 55022, D6700 became 37350.
91003 was actually the first class 91 to be released from works, in April 1988 if I remember correctly. The early pairing of 91001 and 82201 will more than likely have been to do with publicity reasons.
I don't think that the class 89 was regularly or even occasionally used with mark 4 rakes in Intercity days, prior to its premature move into preservation. It was only when it re-entered service briefly with GNER that it saw use with mark 4s. In BR days, it initially worked Peterborough commuter services with one of the short-term mark 3 and buffer fitted HST power car rakes, and later, when the buffer fitted HST power cars and the mark 3s were redeployed elsewhere, it continued to work the Peterborough commuter services with one of the East Coast's mark 2 commuter rakes.
There were also originally thirty three TSO(E) vehicles in the 122xx series. Always useful to have spares I suppose. Although conversely the order for twenty three TSOs (12490 - 512) and eight FOs (11264 - 271) was cancelled when the decision was taken to reduce the length of the trains from ten carriages to nine.
No, quite the opposite in fact: There were enough mark 4 carriages written off as a result of Hatfield and Great Heck that there are now only thirty mark 4 rakes but thirty one DVTs. The Mallard refurbishment sought to consolidate the remaining sets.
Yes. It was in fact 82200 that was involved in the Hatfield derailment.
Yes, the total of 302 does include the DVTs. There are 271 passenger vehicles and 31 DVTs, since the write off of 82221.
There arent enough spare mark 4s to form up a thirty first rake, nor are there any mark 4s that are stored out of service: Every vehicle that can be used, is being used.
Actually, 32 TSOE vehicles were built: 12200-12220/12222-12232. The bodyshell that should have been 12221 was mistakenly fitted out as TSO 12405 during construction, so the vehicle that was 12405 was then converted to TSOE 12232!
I stand corrected on a couple of points then 
I have to admit, that I had simply completely forgotten about the 12221/12232 switch-around. I do recall the later formation of the "Pullman" sets with three FOs and one RFM.
I've no idea why the bodyshell originally intended for 12405 simply never became 12221, as 12221 would never have been registered on TOPS for the vehicle that did become 12405.
That's interesting, as it would have been a massive increase on capacity based on the HST sets the "Electras" were to replace.
I think that revising the "Pullman" sets the way they did gave greater flexibity in set rostering, as the two "Super Pullman" HST sets were restricted to the same diagram day in, day out. The "Tees-Tyne Pullman" set was based at Heaton and the "Yorkshire Pullman" set at Neville Hill. Yet all the Mk4 sets were (and still are) based at Bounds Green, so it would have been awkward having two sets based in London that would have spent every night in Leeds and Newcastle.I had no idea about the original Super Pullman sets, that's very interesting
My 1991 Platform 5 book does show that the original reason for the swap did have something to do with 12204 being unable to enter service in line with the other deliveries of mark 4s, although it doesnt state why. IIRC it also says that 12232 was due to be renumbered 12204 at that time, though of course this didnt happen. 12204 seemed to be out of service for some time: It doesnt appear in the 1993 Platform 5 book, either.
I reckon that they wouldnt have stayed on the WCML. Sixteen sets would be an appropriate number for the Norwich services, comparable to the fifteen class 90s and mark 3 rakes they received instead, but East Coast also has four diagrams right now for HSTs completely under the wires, so they could have put them to some use as well.