Sorry if I let all this time pass but with the exams I do what I can to keep up with the hobbies.
With a little bit of searching I found out that we know almost nothing about the medieval lance, this is the essential extract of all the Robert Jones' book "The Knight" (2012) reports about lance's evolution:
THE LANCE
...From illustrations it can be estimated that the lance of the 13th centuriy onwards was around 14 leet in length. By the 15th century depictions ol lances that were flared and tapered towards the tip appear in the pictorial record, a means by which the lance could be made stronger without a huge increase in weight. The development of the vammplate, a conical dish of metal that protected the wielder's hand, may, like the development of the arret (the lance rest), have had some use in battle, but would appear to be first and foremost a development for the tournament.
This is almost everything you can also find on the subject on the internet. And I have some observations about his words.
In the book Robert Jones says that lances were depicted as flared and tapered, like we can see in The Battle of San Romano
https://en.wikipedia.org/wiki/The_Battle_of_San_Romano by Paolo Uccello.
To me (but I'm no reenactor nor physics) it makes perfect sense that a heavier lance would strike a more powerful punch while a tapered shape would give it a better weight distribution.
It also makes perfect sense that vamplate wouldnt be used in battle since lances were expendable weapons and steel wasnt cheap (
https://www.youtube.com/watch?v=tpbRmnWMXRU as stated in this video).
What I think is strange is that the arret wouldn't be used in battle (the fantastic Knyght Errant talk about its usefulness
https://www.youtube.com/watch?v=bzHK9eGrV8Q), it provides a more powerful punch but I can't see downsides of using it. It being used is also shown in many raffigurations (some of the miniatures you linked too).
Furthermore I don't know how muche the "painters" should be trusted. Minis painters are inconsistent and you can address the straightness of the lances either on the "fast" nature of the depiction or the simple lack of research. But the first reinassance painters like Paolo Uccello may have embellished the equimpent even if as far as I can tell the armours depictions are pretty accourate.
Said that, why don't you think that slightly conical lances were commonly used in battle?