concentrate, chosen

[jiang]

Dear teachers,

I have three questions.

No.1
For days, he put his heart into the rice seedlings in his lab.
Can I use ""concentrated on" instead of "put his heart into"?

No.2
He was elected the president of the union.
Can I use "chosen" instead of "elected"?

No.3
Could you please explain if there is any difference between "illness" and "disease"?
For example,
He has just recovered from a severe illness. Can I use "disease" here?

Looking forward to hearing from you.
Thank you in advance.

Jiang

[Soup]

No.1

If you wanted to, yes, but concentrate on has a slightly different meaning than put one's heart into--there's more heart.

No.2

Chosen to be, yes, but the meaning differs: officials are elected.

No.3

Isn't it because a person can recover from an illness but never from a disease as there is no cure?