Hey good questions.
This is a classic social issue that I won't delve into.
In a paperbased world, people can use whiteout or alter forms or create copies etc.
Or using other office apps you have little control of the form.
With InfoPath you have the ability to add validation and limit what people can do.
The template must be published and the resulting documents are set to use that template from the published location and in the same access method.
You can put permissions on the template and allow everyone to read but only those with permission have the rights to edit or change the template file.
At the end it is just data and can be possibly altered but I think you have more protections then the alternatives.
I am not sure if you have worked with Adobe's forms but they are not as powerful or flexible.
In the case of 1 if they break it they break it. And other policies and practices are in place to catch people who lie, cheat, and alter documents.
For 2 secure the template. it is required to render the data and can be protected from altering.