Unfortunately, after a cursory Google search, I haven't yet found the entire letter or passage that this is taken from. I keep finding the truncated quote that you've offered. But, I have to think that from the word 'constitutions' and the reference to a lack of removal mechanism for judges, that Jefferson was not discussing the U.S. Constitution for three reasons: 1) The plural form leads me to believe he's discussing the several states' constitutions, many of whom preceded our federal document; 2) the U.S. Constitution provides for impeachment and removal of all federal officers; and 3) its not capitalized.
As I mentioned briefly before in an earlier reply, Jefferson and many other Founders feared the Judiciary more than the other two branches, and made sure that there were checks and balances (e.g. impeachment of judges; not being able to rule on a law until a relevant case is at hand) to prevent them from taking over the powers of the other two branches.
Also, be careful when reading passages from this time period. George Washington wrote of a 'lumber room' on the third floor of Mount Vernon, but its not where he stored lumber or firewood. Lumber referred to odds-and-ends, items that aren't used all that often, such as discarded furniture, toys, etc. When Jefferson wrote of the judiciary as "supposed to be the most helpless and harmless members of the government," that is not relevant to the intentions of the Founders (unless you can find the rest of the letter in which he addresses that issue specifically), but rather, he's alluding to the fact that they don't initiate any laws or actions (i.e. its a passive body as opposed to the active Executive and Legislative Branches). He goes on to discuss the lack of a removal mechanism for corrupt judges, etc., which of course cannot be in reference to the federal Constitution since all federal officers are subject to impeachment and removal.
I can't find a context for that either. I happened to stumble across it accidentally and thought I'd post it.
The answer to question #1 is no, not professionally. I do it for fun and personal enjoyment, day in and day out.
Perhaps his letter was to a Frenchman (Monseur), and referred to the US and French Constitutions?
/wild guess