The Taimur is a dead end project. They are trying to retro fit a CX Engine system onto a carcass that's incapable of large megaton loads. They want to put a low signature engine with an effective fuel econonomy system onto a missile system that will be ineffective.
The CX is a fantastic ICBM delivery engine. But, much like the economical versions of cars, the same principal works for ICBMs. Its lighter, less powerful and was designed to deliver cruise missiles to a target, not a thirty foot long ICBM with a nose-heavy design.
An Indian recon aircraft took pictures during out door development and sent the pictures to America which Russia subsequently got hold of. They both claimed that Pakistan's system was ineffective. Whether this is just them putting on a brave face, I'm not sure, but I do know that the CX will not work with that sort of ICBM carcass.
The R7s are now obsolete in Russian ICBM tech, you're absolutely right. However, despite their shortcomings, for two decades they were the ICBM that ruled over the American Minuteman and Peacemaker. Granted, the latter two were faster and easier to retro fit, but the R7 would destroy an area six times the size the largest Warhead on the American ICBMs could cause. Plus, they always had some prepared, so it negated the inefficiency of their fitting, but you're right in that they were the double decker busses of the ICBM family.
Like I said in another post, the Russian Authorities let slip that an Arms Dealer had been caught selling TOPOR ready Frog-7 Launchers as well as the carcasses themselves. Whether or not these are fitted, no one knows, but there was recon footage to back up the Frog-7s, without the TOPORs, however.
You're right, though. If Pakistan acquire up to date ICBMs (or at least make a decent model), we would have more reason to worry. The American 747 would'nt have a hope in hell of taking on a modern Russian ICBM, in fact supposedly in the early 2000s the Russians developed in flight air-to-air missiles for their ICBMs. So its a target of its target.