Making good catnic beam after knock through

[Spark & a half]

Hi Guys,
Any help with the following appreciated:

We've knocked through into another room and have a catnic lintel installed for support. This beam has lots of holes in it.

I'm after the best way to make good, either rendering the beam direct (if possible) or plasterboarding.

If plasterboarding, my main issue is merging it into the existing wall on one side of the room where the beam is inline with the original wall. The plasterboard would I guess butt up to the existing render (sand cement) wall. Will this be ok to do if I chop some of the existing render away and bond the new plasterboard onto the existing wall. Tape the joint between board and existing render and then skim over. (All walls to be skimmed).
See pics for a clearer idea as to what I'm going on about!!

Suppose I mostly want to know... best way to join/butt plasterboard to existing render.

Thanks in advance for any help.

 

[Spark & a half]

Oh one other thing...

If I want to render the face of the pillar (see pic) but not the other sides, could I use 2 x stop beads a few inches apart?

If it was your house and you had a bit of time to put into it, how would you do it?

Help a poor electrician out!!!
Thanks!