The High Altar should be constructed separated from the wall so that there is the possibility for it to be easily walked round and celebration towards the people to be done at it, a thing which is convenient wherever it is possible.
A thing which in the neuter cannot refer in Latin grammar just to celebration towards the people, because celebration is feminine. So it has to refer to whole clauses.
It must refer either to the whole previous bit of the sentence The High Altar ...... at it, or to the ut-clause so that there is the possibility ...... at it.
possibility, not rigid uniformity, is in the mind of the legislator. Otherwise, he is commanding that the the Altar be walked around ... presumably, so as to be censed ... at every Mass; i.e. he is prohibiting the celebration of Mass without incense. He's not. He's just asking that, where possible, walking round the Altar to cense it should not be excluded. (I believe the 1962 Missal, of which I do not possess a copy, explicitly envisages that a priest celebrating versus apsidem should, where this is possible, walk around the Atar to cense it on all four sides.)
The intention clearly is to ensure that in the construction of new churches, liturgical flexibilty is not impeded by the plan of the sanctuary. This passage is misused if it is treated as legislation with regard to the orientation of the celebrant. Or to a prescriptive and invariable use of incense.
I shall not enable any more comments on this detail to my Auctoritas post.
Because, frankly, I regard all this as totally boring and unfruitful. The whole point of my Auctoritas post was to point out that there are vastly broader and more important questions in play here, and that how we worship is not dependant upon how we might be able to extract obiter implications from a legislative text intended for a quite different purpose. For Vincent Nichols or his advisers to drag this into the discussion demonstrates a very silly petty legalism ... and, incidentally, shows the flimsiness of the basis for the onslaught upon Robert Sarah. If Nichols has any concern for his own reputation, he should withdraw his email and apologise to Sarah and sack his advisers.