I think the words βασιλείαν ἀσάλευτον παραλαμβάνοντες ἔχωμεν are rightly translated "receiving an unshakeable kingdom" or "we are to receive [are receiving] an unshakeable kingdom."
(1) I take the participle παραλαμβάνοντες as a participle of grounds, because of Διὸ in Heb. 12:28, which can be rendered: "therefore," "seeing that" or "since." One could of course translate παραλαμβάνοντες as "receiving." But such a rendition is by no means necessary. In fact, Wallace encourages the Greek student "to translate the force of the participle with more than an -ing gloss." So either "receiving" or "to receive" is acceptable from a grammatical perspective.
(2) As far as the time frame delineated by the participle, it must be remembered that we should probably recall aspect morphology here. I.e., the writer's use of the present active participle is his way of subjectively focusing on the action delineated in the account.
William L. Lane, in his Word commentary on Hebrews, writes: "The participle 'we are receiving,' does not express possession . . . but acceptance of a gift or office that is being bestowed" (Lane 47b:484). Lane adds: "The present tense of the participle emphasizes that Christians are now only in the process of receiving this gift and that this process will continue into the future" (Lane 47b:484).
(3) Some have pointed to the eschatological character of Heb. 12:28ff, a point that we must also bear in mind.