I am not a fan of playing founding father ouija as some people are, especially considering as a group the 'founding fathers' had hugely varying ideas on not only the purpose of government, but even what various parts of the Constitution meant. For example, plenty of the founding fathers thought a bill of rights was unnecessary because those rights were implicit in the original document. Clearly everyone did not agree on that, haha.
That being said, the document as a whole is for the most part insanely vague. For example, while the president lacks an apparent limiting power on pardoning, he's also tasked with ensuring that the laws are 'faithfully executed'. Does pardoning people who violate the Constitution violate that aspect of his duties? Maybe! I think for the most part they viewed the Constitution as an overarching statement of principles that would be filled in by laws and norms, a lot like how the UK functions. (the UK has no constitution, it's all basically laws and norms) So, did they collectively or in some plurality think the pardon power should be unlimited? I have no idea. Personally I don't really care either as they've been dead for several centuries and this is a government for the living, not the dead.