I think the angled (90 degree) cuts should not be done using a postform jig if it is solid wood. All the instructions I have found on Google suggest straight butt joins.. not a masons miter which is what a postform jig produces. The masons mitre is great for laminate worktops.. but all manufacturers insist that solid wood must be allowed to move, and a 650mm deep worktop may vary from 650mm by 5mm to 10mm over the seasons.. so a masons miter isn't suitable.
(The grain direction is different on each piece on a 90 degree corner, and wood only moves across the grain, minimal to no movement along the grain)
If using the worktop joining things (in the 35mm holes) you should only use one if possible (on the 90 degree corners).. and a loose tongue placed in a groove if you can. Or else use several of the tightening pieces but cut a wider groove than is needed for the bar, this might allow for some movement as the hidden joining piece can move slightly.
The best way for your straight join is probably to use the 35mm joining strips,.. and a bit of silicone in between... at least you will be able to get a tight join. If one piece is turned upside down, (end to end), and both cuts are done at once then your cuts should match perfectly even if the cut isn't straight, or it's angled slightly. (Angled slightly means not cut at 90 degrees to the surface).
(Edited to add... I was wrong.. the cut must be at 90 degrees to the front edge, if it is slanted then the join WON'T be perfect.)
Movement isn't as much of an issue for the straight join.. because the grain is in the same orientation on both pieces.. and using the joiners allows you to pull the pieces together.
To be honest all the manufacturers have to insist on doing it their way.. in practice you'll often be alright doing it the best you can.
Cheers