Firstly follow company SOPs of course.
If the Captain is doing something which is outside the SOP but clearly NOT unsafe (scenario no 1 I think) then my opinion is that as you have advised him that the approach is outside the SOP you have done your job so far. What you do on the ground is another matter!
If a dangerous situation is developing then, in extremis, you might have to take over but, as has been said, its not too safe to have two pilots wrestling over the controls!
Remember though that if he is working hard (dare I say "maxed out") the first sense which goes in a practical sense is hearing. Quite literally he may not have heard your call outs etc. You now need a "pattern interrupt" - touch him (yes!) on the arm or shoulder to get his attention and repeat your concerns.
Hope this helps!