Upvote:1
For Protestants, and I suspect Catholics too, a doxology (or "word of glory") is often used as a prayer that we can read together as Christians, as opposed to a prayer that one person is making for the group. The Bible has a few famous doxologies:
Jude 1: 24, 25:
24 To him who is able to keep you from stumbling and to present you before his glorious presence without fault and with great joy— 25 to the only God our Savior be glory, majesty, power and authority, through Jesus Christ our Lord, before all ages, now and forevermore! Amen.
Ephesians 3: 20, 21:
20 Now to him who is able to do immeasurably more than all we ask or imagine, according to his power that is at work within us, 21 to him be glory in the church and in Christ Jesus throughout all generations, for ever and ever! Amen.
And I know the Church of England has doxologies in it's prayer book, as it appears that Catholics have their own books. I can't answer your question in the form of the three questions you've posed, but it appears that at it's heart, a doxology is just a prayer that we can use or not, as we choose. Whether we use a doxology or not, our heart's condition must surely be more important before God than the choice of Words.
Upvote:2
and 3. I guess it has just historical reasons, and it will probably be corrected someday. In most languages, there's just one version of Gloria Patri.
In Liturgy of Hours it should still be Gloria Patri, and if you pray it by yourself, the translation shouldn't be important. On the other hand, it shouldn't be replaced by other, though similar, prayers (but I've seen much bigger tresspasses against liturgical rules and AFAIK it doesn't make the whole prayer useless). In Rosary and other uses, it's even more free.