Thank you, Twitch! Like Henry VIII, I am called to defend the faith; except that he was a schismatic and heretic and is burning in hell right now while my date with the flames will have to wait a while longer, it seems.
On the other hand, he was able to kill a cardinal (Fisher) and Thomas Moore, while all I can accomplish is to piss people off here. Speaking of which, Cofty, how's it going?
You said, quoting from a Catholic source:
However, for those who knowingly and deliberately (that is, not out of innocent ignorance) commit the sins of heresy (rejecting divinely revealed doctrine) or schism (separating from the Catholic Church and/or joining a schismatic church), no salvation would be possible until they repented and returned to live in Catholic unity.
Correctamundo. However, the key to this sort of thing is whether people are working out of ignorance. We have pointed out that a sin requires that the matter be important (which schism certainly is) and that it be done with full knowledge (which is often not the case). For people who were afflicted with JW-ism or other Protestant fan-fictions, we might suppose that they have not committed the sin of schism simply because they don't know what they are doing. Hell, if everything the JWs told me about the Church were true... well, I trust you get my point.
That said, you point out that both the Eucharist and confession require apostolic succession, which my Protestant separated brethren lack. Thus, they cannot receive these sacraments.
For a Catholic who becomes a JW, the question is somewhat more complex. It cannot be denied, after all, that the religious education of Catholic young people is something like a catastrophe, allowing silliness like JW-ism seriously to be entertained by some Catholics. Their culpability is unclear to the extent that they have no idea what they are talking about in the first place.