When is it optimal to choose a lower favored character/kart/glider? Such as a high end (purple) in the bottom row vs a normal (silver) in the 2nd or 1st row

There’s no way to know for sure without knowing what the combo time boost is from the favored level. Once you know that, just compare the bonuses.

The time boost on the gliders is almost ALWAYS better than not having that bonus. It extends combos and brings you into higher scores.

With Gliders, it’s all about combo time. If you have a high end tier two glider that is Lv 2 or 3 it might outweigh the tier 3 boost. Otherwise stick to tier 3.

I'd always go for a more favoured glider because it's easier to maintain combos and the combo bonus is higher, and the most favoured character for more items. The level bonus only applies when the component is on its favoured course so the level can usually be ignored if it isn't favoured.

For karts, it could be worth using a lower one if the course is very short and your less favoured kart has significantly more base points than the favoured one, although I think this would only happen in very specific cases

Here's my example: Say you have a Dry Bowser with no EXP. During the Halloween Tour, I would opt to use him to max it's EXP and thus boost my rankings for the Tier system. Same goes for the Warship, etc etc.

Afterwards, I'd just focus dumping EXP on high-ends > rares > commons.

Once that's capped, have at it and beat your own scores.

It’s almost never worth it, except if you got the ch/kart/glider lvl 4+, then maybe is worth it.