It sounds like metacognitive therapy is just one of the regular parts of cognitive therapy - to realise that your thoughts are inconsequential by probing them intellectually - but by focussing on those types of logical questions you're no more highlighting and enlarging your fears than you would be using exposure therapy; at least to me. Also I knew my thoughts were grounded in irrationality, trying to deal with them using rational questioning was to me always pointless because my thoughts never would bow to sound reasoning.
It's probably one of those cases where different styles of therapy are better suited to different people - the trouble is anyone with OCD will always think their current therapy isn't working for them because we're all pretty pessimistic so it is one of those things that is hard to objectively judge by yourself. If you're surpressing your thoughts, then it seems like your brain is working against the therapy (not something you consciously want, but just a result of the fact OCD is slippery) - I'd say exposure would now be better as that would bring the thoughts to the fore of your mind and allow for more confrontation. However in time I'd imagine you'll doubtlessly find away to focus on your thoughts mentally too, so it's up to you. Personally I always tried to do what would make me feel "worse" (more at odds with the safety seeking behaviour), whether that was mental or physical.
I think "what ifs" are what typifies any form of OCD, say when someone who is worried about contamination will think "What if that spilled drink causes people to contract dysentery and die"; however I think you're correct in that Pure-O does focus on existential anxiety which is why Pure-O types don't normally have overt complusions. From how you phrased your response, it sounds like you think that by building better life quality it will assume the role of mental rituals in protecting you against harming yourself as you would feel like you now have "more to live for"? I put this in quotes not to be demeaning but to highlight that having more to live for is intimately linked with your own perception, even if you do have more to live for by whatever arbitrary standard you chose to make (wealth, friendships, love, status) you won't
feel like this is the case whilst under the sway of OCD. Also surplanting mental rituals with a better quality of life doesn't hold under scrutiny as it implies that OCD is solvable as long as the right external factors are in place, when really the problems only exist in your mind. As an anology to my own problem, I thought well maybe if I feel closer emotionally to my partner I'll have less thoughts about harming her - but it just doesn't work like that. Of course though, improving quality of life is its own reward.
As to taking antidepressants, I tried citalopram but it didn't work for me and seemed to make my anxiety worse. There is a big caveat to this however in that I was also getting drunk on a regular basis as well - I don't think the two mix well at all. If I had have stopped drinking it may have worked like a charm, but that's the beauty of hindsight. I'm doing a medicinal chemistry degree and have in fact written an essay on SSRIs like prozac, sertraline and citalopram (may have different names depending what side on the pond your on); so I know for quite a lot of people they do work. I'd try which ever your doctor thinks would be applicable to you (if they do think one is) as they may help your progress. However, you should be prepared that you normally have to take them for 6 months minimum before your doctor says that you can get off them again - something to bear in mind if they don't work as you expected.
I don't check these forums too often, so don't be surprised if sometimes I take a little while to reply. I guess as a closing comment if you think that taking a course of action or thinking a certain way will fix or help your OCD which is not dealing with exposing you mentally or physically to your obsessions, don't do it. All it will do is make your OCD persist longer, because as I said there is no solution just an aclimatisation to the thoughts so they don't trigger anxiety anymore. In a different thread to someone else I said that
Tormenting Thoughts and Secret Rituals was a reasonable book for reading about OCD; in that as a suggestion for Pure-O people it says that taping your worst fears in as much explicit detail as you can manage and then listening to them on repeat for an hour every day is good exposure. I never tried it personally, but it might be worth a shot and if it didn't work then you can always try something else.