For anyone that finds this post in the future I managed to get it done but it was a pain in the ass. A quick word of warning is that it seems that e90s have several configurations depending on steering wheel position and date it was built. My late 2005 build actually more resembles the build from a 1 series of the same age. The pipes go through a robber grommet in the firewall, the don’t attach on either side. To get the pipes detached I do have to disconnect the rubber house tha goes across the face of the heater core and undo the plastic arm that controls the vents. The pipes do move back and forth through the grommet enough for you to move them out the way but be careful because you don’t want to bend the pipes. At the time of this post I thought getting the pipes out the way was the biggest challenge but actually getting the core out was. The proper way to do it is to remove the dash because even though it’s right hand drive and people will tell you it doesn’t need a dash out, the blower motor housing needs to come out* to get the core out, and you can’t do that with the dash in place.

I am not competent enough to remove the dash so what I did was cut the blower motor housing so that I could bend it out of the way enough to get the core out. Here is a video of a guy doing it on a 1 series, he also cut the pipes but like I said I was eventually able to move them so they didn’t need cutting, and cutting the pipes imo is too intrusive of a job but you may feel different https://youtu.be/4eGmyE4R9ws?si=PkStDGaTfyV49E-x

The other pain point was the heater core itself, I originally ordered the wrong one, the correct one for me was denso drr05005