Given the subsequent replies to this suggestion, I propose a compromise that I think could suit everyone.
OK, instead of all us joining a large existing party, how about we instead join a small existing party? Doesn't really matter how moribund said party is, because our politics (if they're any good) will bring vitality to the organisation.
That way, we get the benefits of whatever national/international infrastructure they might have (spaces, equipment, etc), without being subject to the discipline of a mainstream party (because they will purge the fuck out of a red if they catch wind).
If there's some aspect of the existing party's politics we don't like, we keep the disagreement civil (takes practice but is doable, most politics is just the art of not nibbling b8), but firm, and make sure that our own further recruitment efforts trend toward our side of the debate. If/when our recruiting efforts pay off, we'll end up with a majority in favour of our viewpoint, and will have established a tradition of comradely debate around points of disagreement instead of Life of Brian-style splitting at the first sign of trouble (as seems to want us to do, and as notes has already happened w/the same organisation mentioned).
Getting together an actual party would be helpful for another reason I can think of, too. There are a lot of ideological debates on this board, but having an actual organisation would help to clarify some of those debates, since people would be forced to put their perspectives into actual action instead of disappearing into their own navels, as people are wont to do when they have no hard reality on which to test their ideas.
I note that all of the infiltrations you describe were right-wing in character. I think that such a strategy is effective for right-wingers, since moving any organisation rightward is 'going with the grain' as it were, in that it aligns the organisation more closely with the awfulness and brutality that is naked capitalism.
As a final note,I want to add that I think internal party democracy is the key to avoiding rightist deviation and opportunism. The failure of most left-wing parties attempting to participate in parliamentary politics is that they end up tacitly accepting a situation where one person has disproportionate power (because you can only have one/a few parliamentary reps). Their goal becomes retaining their power, which requires accommodating the demands of the bourgeois state.