The rule about simultaneous is from
HTA regulation 626 is:
"(3) No traffic control signal system shall be operated so as to show more than one circular indication simultaneously on the same traffic control signal. R.R.O. 1990, Reg. 626, s. 1 (3)"
So yes it does seem legally possible to show a red ball and green thru arrow at the same time. But personally I think that display would be very confusing to most drivers since we're mostly accustomed to the red ball controlling the thru movement. I prefer the display that's used in Ottawa, with a separate head that basically acts like a right turn lane signal. It's aligned with the right turn lane, so I think having a separate signal head is actually a feature, not a bug.
Ontario is the only province that explicitly forbids red arrows. There are numerous red arrows in other provinces, mostly in Québec and Alberta.
Montréal QC has lots of red arrows. For example at
Rue Notre Dame & Rue Frontenac
View attachment 622307
Canmore AB has at least one intersection with red arrows:
Bow Valley Trail & Railway Ave / Benchlands Trail
View attachment 622306
Calgary AB does indeed use red arrows as well. Like you said, they use them in combination with a red ball to indicate "no turn on red". Here's an example at
Richmond Road & 17th Ave. I couldn't find a Streetview image with the red arrow but you can see it on the sign.
View attachment 622305
That intersection is in Gatineau QC, not Ottawa ON. The intersection on the Ontario end of the bridge has red balls instead of red arrows.
The typical interpretation has been that the ON HTA doesn't apply on federal roadways, including all National Capital Commission roadways (including but not limited to interprovincial bridges). This is why the NCC installs federal black speed limit signs instead of Ontario / Québec standard signs.
I personally don't understand why federal ownership would exempt a roadway from the HTA but if that is in fact the case, then red arrows could be installed on any NCC roadway, not just those in Québec. The signals on the Ontario end of the bridge seem to be following the City of Ottawa's standards, which are based around the HTA, which is probably just for consistency with other signals in Ottawa.