(also called Rhumes palmata, or Tibetan rhubarb) was the laxative. It has even more oxalic acid in its leaves than Rheum rhabarbarum, so its stalks might not be edible at all. It was the root, as you say, that was traded from China. In Chinese traditional medicine it was ground to a powder - although English druggists would have mixed it up in the ways they thought most efficacious - using alcohol to make a tincture was very common for lots of drugs.
Nicholas Culpeper claimed in his 1653 herbal that there was an English Rhubarb that was 'nothing inferior to that which is brought out of China' - but I am not sure if he is identifying a type of rhubarb in his description.
The edible variety (Rheum rhabarbarum and similar species), that I think of as so quintessentially English, was introduced to England in the later part of the 18th century, first grown only in the mild southern climate and only offered for sale in England toward the end of the first decade of the nineteenth century. Like apples, rhubarb seeds hybridize easily, so to grow a plant true to type, the rhizomes needed to be transported and nurtured by professional nurserymen. And of course, it is very astringent, and not very edible without sugar - which was being blockaded by abolitionists at the time it was introduced.
Still, as it was very rare in England after Susan had been offered to Crosby, and is only mentioned in its medicinal form, I can't read it as a fling at the slave trade.
As a fling at publishers like Crosby, however, I think this passage might be one of many (Unfortunately, I only clocked onto this angle last week, or I would have made it my focus for the entire novel)-- publishers who made money out of pandering to a public taste for horrors.
One aspect of this passage that I am not sure of: Did Miss Austen believed that Catherine’s second thoughts on the subject were simply true, or was she making a more complex satire on the unequal application of the laws of England?
For example: "in the central part of England there was surely some security for the existence even of a wife not beloved"
While Justice Sir Francis Buller might not explicitly have suggested anything as ungallant as the 'rule of thumb', he did maintain the legal position that by being married to the victim, a man could be found not guilty of murder in circumstances that would have made him guilty of murder if the victim had not been his wife, and that legal position was not changed in Jane Austen's lifetime.
In fact, in 2009, whether or not to remove the 'provocation' defense is still scheduled for debate, and is still not repealed. I am not at all sure that a wife has the protection of the law in this regard now, and wives are no longer regarded as their husband’s property, and maintain their independent existence in law now.
It is true that servants (unlike wives) were not chattel slaves in Jane Austen's time (that is, not in the main part of Britain), but bonded slavery was alive and flourishing at the time, and the inhuman working conditions and less than subsistence pay, and range of punishments an employer could mete out, were some of the reasons the Luddites were so close to open rebellion at the time, and if Eleanor was aware of this point, Jane Austen must also have been.
Thomas de Quincy's Confessions make it clear that poisons and sleeping potions were available from every druggist. This is true for our time as well. While he could have procured poisons like tinctures of mercury, cyanide, arsenic and foxglove, that you would not find now, Mr de Quincy would probably be impressed with the much stronger and more reliable opiates, and the battery of remedies for the toothache, sleeplessness, anxiety, bad dreams and biliousness he suffered, that are available (unlike rhubarb) from every druggist nowadays.
While the rule of law seems to have held up generally, I am not sure Catherine’s second thoughts are more sound than her first.