1. Based on your travel you will need 7 LHR-DEL in J round trips to get QF Platinum, or 5 LHR-DEL J round trips and 4 Y roundtrips Uk-Europe (greater than 1000 miles each way). Sounds like you won't have a problem.

2. As far as I know QF won't comp you to Plat just because you have BA Gold - but I may be wrong - you may want to try by showing them your travel plans for the upcoming year.

3. In terms of the transition, I would suggest doing it from the beginning of your BA EC status year - that way you keep your emeral status on BA and enjoy the benefits of OW Emerald while you attain QF Plat.

4. Bonus miles - assuming you won't be comped, you will very quickly get to QF Gold (3 LHR-DEL J roundtrips) which will give you 50% status bonus - what you get as BA Gold. So your miles opportunity cost will be marginal.

5. The 4 QF segment rule is very inconsistently enforced, and many FTers have stated that it isn't. If you want to be safe, only QF operated flights qualify for the 4 segment rule (The T&Cs state "On Qantas or wholly owned subsidiary" but I believe it has been interpreted to mean QF metal).

6. On the other hand you can enroll in AA Plat Challenge and just after one LHR-DEL J roundtrip you would attain Plat and the 100% miles bonus. You could then go on to qualify for EXP as usual (again you won't have a problem). Only thing against AA is that any transatlantic flight between Uk and US on BA won't count towards your miles or status - ie you are forced to fly AA transatlantic.

7. The major benefit of AA EXP is the upgrades you receive every year (which apparently are much easier to use than QF UCs).