it seems that I could gain a fATPL for less than £25,000
How on earth have you come to this figure? An instrument rating alone is almost this much, you will be looking at around double from where you are now to an fATPL.
to answer your questions:
1) No, you can do CPL either in multi or single engine.
If you do single engine CPL you then do multi engine course (6 hours) followed by multi engine IR. Otherwise, you don't have to do the multi course and go straight onto the IR. You will have to do the sums yourself as to which suits your budget, as the CPL and IR have different pre-entry requirements so some find it advantageous to do Multi-IR before their CPL.
2) Yes, it has to be a multi engine IR to be valid on multi engine aircraft. You could do a single engine IR then convert to a multi, but it would probably cost you more to do it this way, and if you have the multi it is valid in a single anyway.
3) if going modular, I don't think anyone cares. I did CPL then instructor rating, instructed for about two years then did the IR when I could afford it and am now quite happy in an airline job.
4) it's up to you how you do it. They have different entry requirements, you could do IR first with I think something like 70 hours (check LASORS) where cpl needs 150. You still need 200 hours total and 100 PIC to get the CPL issued, so overall the end hours should be the same.