In practice, there's a wide gulf between what the Church formally establishes as dogma and the adherence of individual Catholics. It's generally resolved as the Church holding the door open for forgiveness through a person's entire life, as long as penitence is a possibility. It's rare for the Church to excommunicate an individual Catholic (or even deprive them of communion) even for very public statements regarding their support of abortion.
Sometimes, the door is held open so long the Church's view on the topic changes and forgiveness is no longer needed to align the practitioner with the Church's stance.
No, you can't, and nothing at your link supports your claim. Support for legal abortion (or the non-criminalization of anything; criminalization is not symmetric and there are cases where this wouldn't be the case for supporting criminalization of an act) is, where not obligatory, inherently a prudential judgement as criminalization always inflicts harm intentionally.
Moral support for abortion would be dissent from authoritative teaching, which is of a more serious (both doctrinally and in canon law) character, but that's an intensely different thing.
No actually, you can't. You can fit the broader definition of Christian but if you hold these views and profess to be a Catholic, you are in fact a heretic and can be excommunicated[1]. I see this fiction posted again and again on HN, and while it's not my intention to support or oppose the Church's viewpoint on this people should be educated and aware that they can't claim to be certain things if they aren't them.
[1] https://www.vatican.va/archive/cod-iuris-canonici/eng/docume...