The fact that you are approved for your OPM retirement will not have any meaningful effect on your OWCP entitlements. You can elect to stay on OWCP for "as long as you can stand it" or until OWCP kicks you off for some reason. OWCP will not kick you off its rolls solely because OPM has approved your retirement.
I have only had one or two people go the Voc Rehab route and they both hated it. However, they both really had no idea what they wanted to do, resented the whole idea and thought they were not able to work (and they probably could not). Consequently, they pretty much let the counselor run the show. And they got railroaded.
You seem to think that you can do some work. The counselor probably is used to seeing people who really don't want to be there and have no idea of what they would do regardless. In other words, the counselor quickly figures out that the person is "going thru the motions". He or she probably responds accordingly.
I am not saying that it will work, but if you can convince the counselor of your sincerity, you might get what you want as long as it is not "too extravagant" or unrealistic for some other reason. Think about what you did when you were trying to persuade others to your point of view. You probably are going to have to be a real salesman and the person you are selling is yourself. I am not suggesting that you "drop your guard" when dealing with the counselor. Just be prepared with evidence that this particular career choice is a good one for you given all of the surrounding circumstances. By all means, do those voc tests. If you really do have a choice that resonates with you, the chances are good that the voc tests will validate that choice.
When I say not too extravagant, OWCP is unlikely to approve a training program in excess of two years.
The other thing that I suggest is that you research the labor market for your career choice. The counselor is not going to recommend, for example, that you become a printing press operator, since that particular occupation has been rendered obsolete with the introduction of computers and related word processing software. But, if you can provide evidence that your choice has lots of job openings that would be a real plus.