can I use "used to" for something is "used for" some objective?

Solution 1:

Al of your sentences are correct.

Note that in your (a) and (b) sentences, 'The computer is used' is the passive of 'X uses the computer'. It has nothing at al to do with the 'used to' for past habitual action.

Solution 2:

Your example 1 would be pronounced 'Yoost' Example 2 as 'Yoozed'.
It's quite difficult to confuse the two given the context of the whole sentence, even if both are in the same sentence.

'I used to be terrible at spelling until I used a dictionary'

As for used to Vs used for, both are ok unless there is ambiguity, i.e.

The greyhound is used to running. It is familiar with the act of running.
The greyhound is used for running. Running is the dogs function.

Both versions of your Bivariate sentence are contexually and grammatically correct so it's down to personal preference.