My flow rate stated to slow a couple of months after surgery and I had about 13 procedures in three years 9 of which were dilatations. At worst the flow was down to 0.5 mls per second but I would have to perform self dilatation. Generally I was back into theatre before it got down to 5 mls per second sometimes I did get down to about 3 mls per second.
Prior to RT my flow was probably between 5 and 10 mls per second so the flow rate was not an issue but the consultant's concern was that RT would affect the scar tissue and close the urethra. His first option was bladder removal, no thanks.
I did have one occasion where I was in full retention but that was following a dilatation and I had a urethral Catheter fitted in s treatment room no anesthetic, it was uncomfortable but bearable.
The SPC fitted pre RT had a flip flow valve fitted between the catheter and the bag in an attempt to keep the bladder working normally, so yes I could urinate through the penis with the SPC in. The RT did cause my urgency to get worse so having the valve and bag did mean I could just open the tap instead of leaking.My flow rate has increased since the RT and is between about 6 and 14 mls per second.
Mine is a bit of a horror story and probably not the norm.