Anyone knows anything about the pre-V2 regulations/rules/opinions on performance in churches? I have heard the common practice is to move the tabernacle or the Host to the side which is totally absurd to me. Also it makes me cringe of the whole idea of standing in front of the altar, facing to the audience to perform. IMO the churches are consecrated and it serves only one purpose which is to worship God. But I also heard that it's done this way pre-V2 too so as long as they're not bad songs it's fine so I'm not sure if I was wrong. Also how about performing in protestant buildings? The whole idea makes me cringe too though the Host is not there.

I have heard the common practice is to move the tabernacle or the Host to the side which is totally absurd to me. Also it makes me cringe of the whole idea of standing in front of the altar, facing to the audience to perform.

You are describing the typical novus ordo "mass."

Prior to Vat II, if something non liturgical was going on in the church, the Blessed Sacrament was removed to the tabernacle (Altar of Repose) in the sacristy, the sanctuary candle was extinguished and/or moved to the new location for the duration of the event.

I see, but how was "something non liturgical" allowed in churches? What are some of the examples?

Sounds like you have good Catholic instincts, P3.
Canon law (1164) is actually concerned even about the basement being used for purely secular purposes:
https://archive.org/details/1917CodeOfCanonLawCommentary/page/n2407/mode/1up

Thank you. So from the article it's saying if a place is merely blessed rather than consecrated it can be used for worldly purposes. But that clearly doesn't apply to the church hall itself as I assumed that the church hall was usually consecrated?

To give an example I came across today, this is classical music performance, but secular, and they were dancing in the sanctuary! Would the catholic church before V2 really permit this?

The procedure described by Miseremini is correct. 

Sacred concerts were often held in the larger, more prestigious churches. The work had to be sacred, so as to be fitting for the sacredness of the space. Never secular concerts or events. So, an interesting example: Verdi's requiem was performed at San Marco's in Milan in a concert, not liturgy, since it was deemed too obviously operatic (even in those days!) for liturgical use. Nevertheless, it was a sacred work since it treated the Requiem Mass texts. Another example of a sacred work that was not fitting for liturgical use (due to its scale) was Beethoven's Missa Solemnis.

Most major French churches had regular organ concerts on Sunday afternoons and still do to this day.

And other magnificent works of sacred music that were not fitting for the liturgy may nevertheless have been written for devotional purposes, e.g. the many beautiful settings of the Stabat mater, the seven last words of Christ, etc. In these cases, the Blessed Sacrament did not need to be removed since the music accompanied a devotional practice and was not simply a concert. Of course, we can be sure that the "cultured" yet impious would have attended these devotions merely to listen to the music.