Taking them in turn:
You will need to have either the ME type on your licence or have completed an approved course (IE initial ME type rating is 8 hrs. Having completed the course you don't need to take a skills test and then pay to add the type to your licence in order to complete the IR. Works well for N sea guys you train on an As355 but never intend to actual use an As355 IR (because IR's are type specific). As pointed out, under grandfather rights you can get round the TRT issue if you use Bristows AB206.
That said. The last I heard Bristows were having some maintenance difficulties with the AB206 so a call to their Norwich training school would be a good idea.
Under JAR a SE IR (from a licensing point of view) does not exist. So at the end of the Bristows course you get a certificate of course completion - practically an useable piece of paper unless you find a company to pay for the ME upgrade (or pay for it yourself). Costings wise if you're paying yourself it is cheaper to go ME from the outset.
Your exam credits are valid for three years up to date of licence application. So you'd need to have finished the course and have been tested before the theory credits have expired. The CAA have been known to extend exam credits but you'll need a good reason for them to consider it (Ie one outside your control - and no course availability isn't viewed as outside your control
. Sadly just running out of time probably won't be good enough.
It probably won't help if you're short of time till your theory credits expire but the last i heard most training providers are fully booked till atleast early 2007 (in some cases until mid 2007) and have lengthy waiting lists should any student drop out