So with all the advance planning to do for the next sector, I presume the PF has to still keep an eye on the navigation and monitor the systems whilst the PNF covers the other tasks in hand?
It's not quite like that Egon. With experience, planning for the next sector amounts to no more than coming up with a fuel figure based on weather, any RTOW restrictions and expected delays if any. However, just like the proverbial iceberg, there is so much more going on below the surface. This is why I say line training brings a very steep learning curve. And even when you've finished, you'll look back on yourself after six months to a year flying the line and realise just how much you thought you knew but didn't! It's all about a solid foundation of training, and then experience.
In very basic terms, PF will fly the aircraft, either through the autopilot or manually, PNF will "do" the radio, fill in the flight log and get the wx when required, while monitoring PF's actions.
Have just skimmed through "Map Shift" and IRU alignment loss; do you have time to keep a constant Plot going to confirm the position of the A/C or is it a case of juggling the tasks and including the EHSI in an instrument scan?
Full IRS alignment loss would be an extremely rare occurence. No plot is needed - that's why we have map mode. Generally most operators require one position check per hour and before descending towards terrain to ensure position displayed on map is actual position. This is easily done using a conventional VOR/DME position check, and comparing it to the position displayed on the EHSI map.
The full complexity of a commercial operation shouldn't be underestimated, and this is where experience is needed - to anticipate problems coming up, and to take action before they occur. This is often why, if you observe an experienced crew, they appear to make the job look a lot easier than it actually is.