There is an item you can buy from the store that doubles your health as long as it's equipped.
You should have enough drops to buy it just before the first world's boss. It seems to be intented to be the game's unlockable easy mode as x2 health is more than enough in most cases.

Only accessibility option is to add white or black outlines to sprites.

No clue about the game's length yet. The first world took me about two hours including the unlockable chalenge level

Depending on your skill, the 100% clear time should be between 10-20 hours I think.