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If one were to look at the keys from the piano's side, should the keys be level (parallel to the ground) or should they slant inwards inside the piano (front end of keys higher than back end of keys)? Thanks.

Typically, they slant towards the piano's interior.

The tops should not be so high as to result in the balance rail pins being buried in the key buttons.. They should protrude at least 1/16". But not so low as to have no room for proper key dip, 7/16". Or whatever results in proper aftertouch for a given blow and let off. The keytops should not be higher than the cheek blocks. They should also make a slight wide rectangle with the key slip. If all those things are met, it doesn't matter if they slant.

the slant of the keys is the "attack angle", it relates to the horizontal in different ways, some keyboards being horizontal at mid blow, other at full dip or almost.

too much slant is often noticed on small verticals..

the slant influences the way the leverage varies during the stroke (between key and whippen).

so the intended height for a given keyboard is possibly the most important dimension in regard to the original action design.

not always really easy to deduce, but there are different points that allow the tech to know he is right, if he does not have the factory dimension.