. But I know you cant tint with normal tint on the inside of a double pane window. .
That is %100 incorrect. You can absolutely apply window film to the interior side of a double pane (sealed) unit. We do it daily as we are an authorized 3M window film solutions retailer (in Canada). There are some MAJOR factors that will determine which films you can use but it is absolutely possible. As well you can't install car tint onto sealed units... that will most definitely cause thermal shock. **now in saying this, if the unit is tempered you can get away with it as tempered glass can handle a larger psi load. I would still be weary about installing car tint on a tempered inbound lite - my recommendation would be to stick with commercial tint. I would ask the questions about issues with thermal breakage prior to purchasing the film from your distributor prior and if they say there is no issue I would move on to another distributor as there are risks.
As far as applying the 40c on the exterior of the window, that will not cause a single issue. Make sure to clean the window properly before installing and get to it. There is no chance of thermal breakage by adding a print to the exterior (1st surface).
For the heat reduction, you will notice that it won't get as hot as quickly as the film will help absorb some of the heat, but as soon as it starts to heat up it will start to pass the heat inside. That's the same misconception with blinds... blinds do not keep a room cooler, they darken a room so to the mind it feels cooler, but the blinds absorb the initial heat but once they heat to a certain point they just start to pass the heat inside.
All in all just make sure to do your homework prior to suggesting a certain window film for heat/glare reduction.
Chaz