Suffice it to say

In Bryan A. Garner's "Dictionary of Modern American Usage", the set phrase "suffice it to say" is explained to be the subjunctive form of the indicative "it suffices to say". The article on the subjunctive mood in the same reference claims that, although its use is less common in English today than in the past, the subjunctive survives today in six contexts:
1) conditions contrary to fact
2) suppositions
3) wishes
4) demands
5) suggestions
6) statements of necessity

My question has a few parts:
1) Was there a time when the subjunctive was used in English for some other purpose than the above mentioned six? If yes, for what purpose(s)?

2) Does the fossil phrase "suffice it to say" conform to one of the above six? If so, which one? I am having trouble discerning this with any certainty. I was thinking it might be 5 - that the speaker is suggesting that it is sufficient to say "X" about the complicated matter "Y", i.e. there is the tacit idea that much more could be said about "Y", but the speaker is opting for the shorter and more simple "X". If this reading is incorrect, then in what sense is it subjunctive?

3) Would it be foolish or bad form for me to have some fun with this construction and use some verb other than "to say" in this archaism, e.g., "While he could squash the offending spider on the windowsill, suffice it to open the window and shoo the creature out."? Or might this just confuse people and come off as bizarre, rather than, say, playful?

Thanks very much for any efforts to clear up my confusion on these points.

Regards,
K