Learn English – Can “I be” ever be considered correct

Purely in terms of the terminology, then a distinction sometimes made is that:

• inflectional subjunctive is the type found in Old English, German, modern Romance languages etc, in which verbal inflection distinguishes subjunctive from indicative;
• periphrastic subjunctive is the type found (if you adopt this analysis) in modern English, in which subjunctive is distinguished from indicative by way of modal auxiliaries/other verbal constructions.

There's really no consensually agreed upon "wrong" or "right" answer to the question of whether English actually "has a subjunctive". If you adopt the analysis that "subjunctive" is the grammaticalisation of non-assertive force with a verbal paradigm-- which seems to be a close approximation to what the phenomenon is in Romance languages-- then it's fairly clear that English doesn't have such a phenomenon. (Saying that English has a past subjunctive on the basis of the form "if it were" is a bit like saying that English is a verb-final language on the basis of a phrase such as "Language does not a society make": it's proposing a paradigm on the basis of a rare exception.)

If you extend the definition to cover cases such as English "It is sad that he should leave", "David commanded that she leave" etc, then there are various issues to be considered which are typically glossed over in language learning textbooks:

• care must be taken to recognise where the similarities and differences actually lie between these phenomena and the inflectional subjunctives of French etc;
• it is worth thinking about what the motivation is for proposing "subjunctive" as a 'special case' of modal verb usage.

The sentence is grammatically correct. "your ensuring..." is simply a very long gerund phrase, and can act as the complement to "We appreciate" in the normal manner.

However, as you say, the sentence could also be written "We would appreciate it if you ensured..." while remaining correct. (Note that the meaning would change slightly, as the altered sentence implies that the ensuring has not yet happened, while the original implies that it is over and done with.)

The altered sentence would simply be a conditional statement reversed from its usual order. Normally one would cast it as a conditional the following way:

"If you ensured..., we would appreciate it."

However, reversing the order of the clauses changes neither the meaning nor the correctness of the formation. Note that, as you suggested, "ensured" is in the subjunctive mood here. However, "your ensuring" not in any way subjunctive. Subjunctives are formed by using the past tense of the verb in question. "Your ensuring..." is a gerund, formed by using the present participle.