"Verbs with infinitives in "-ąć" change "ą" to "ę" in all forms except the masculine singular."
Swan, Oscar (2008-10-12). Polish Verbs & Essentials of Grammar, Second Edition (Verbs and Essentials of Grammar Series) (Kindle Locations 1342-1344). McGraw-Hill Education. Kindle Edition.
We is feminine in this sentence? What would the sentence have been if we were masculine?
I understand you got this as a Polish sentence that you have to translate to English? Because that's how I see it opening this topic. If so, that is just the authors' choice.
For masculine group (or a mixed group) it will be "Zamknęliśmy walizkę".
Is it a fault of the TTS, or you do have to pronounce the 'ę' like 'e' in "zamknęłyśmy"?
Yes, pronouncing ęł would be really too much, eł will sound a lot more natural. Or you can nasalize it just a bit.
Yes, and both are accepted here. Are you saying one of them should not? Because I don't see a problem with any of them.
I talked to Immery about this today. The category of this lesson is called Past Perfect. In English, "we have closed," is present perfect. Past perfect is "we had closed." They should've named this category Past Perfective. English doesn't have this. It is just simple past tense with a verb specific to the action. If that's the case though, then past perfect and present perfect in English are analogous to imperfective in Polish (and Russian, for that matter).