The entire way takedowns are considered in the MVP part should be changed. Instead of a raw number of takedowns, it should use the XP value of those takedowns (as that measures effective gain compared to laning for XP, and on that would punish the no gain brawling that often enough happens).
With it, a death should apply negative value to that. Even if such death isnt effectively causing a death timer (mech or misha dies). As in the end, you are still granting that XP to the enemy.
For siege, healing and hero damage. They should generaly be consided below XP. Since a very efficient killer is often going to end up with less hero damage, which would then be counter productive.
Healing on this is a more special case, as we want to reward this better because healers already have less direct influence. Siege however is a very static value, and getting higher values is nearly always better. So i would say this becomes a very static + value.
For hero damage however, you can effectively feed the enemy a lot of damage, and it might be because you have very good sustain, or that your healer did a good job. And i think this is the part where we can balance these numbers so hero damage becomes a less significant modifier (we have takedowns already after all).
For hero damage i would suggest a system where the damage you took is considered equal to damage you deal, in such way they cancel out. It doesnt need to modify a lot of the other scores, but it can sort out those who deal a lot of damage safely.
However, there are heroes that generaly are going to take more damage like tanks and bruisers. And they should be compensated on that. Self sustain healing can compensate the damage they received, but for some heroes that doesnt work evenly.
Sadly i dont know a perfectly balanced system on this (or even have good ideas), but i think this sort of approach would be better.
In the end however xp and siege can be used in a quite static way. As these values effectively are reliable. Takedown related xp on this can just be added to the xp value.
Last thing though, i think that the MVP screen should not realy pick an MVP, but instead aim towards showing commendations for each player, where some are simply just either given or not, while most are in a gold/silver/bronze format where depending on the game either just 1, or all 3 are given out.
A player which then collects 8 is generaly going to show better than one who just had 4. People can just vote whoever they think is MVP. And depending on the medal system, those medals automaticly already count as a vote to decide the MVP in the end.
But thats only relevant because they want to show the amount of times a player did get MVP on their profile. Which i think isnt realy useful anyway.