Here are the corrections for each of the sentences:

1. The board of directors **is** planning to implement new strategies next quarter.
=> **Correction:** Change "are" to "is" since "board of directors" is a collective noun treated as singular.

2. If they **had** known about the changes earlier, they could have prepared better.
=> **Correction:** Change "would have known" to "had known" for the correct conditional form.

3. The report was submitted after it had **been** reviewed by the committee.
=> **Correction:** Change "being" to "been" to use the correct past participle form of the verb.

4. He claimed that he **had** completed the project by the end of last week.
=> **Correction:** Change "has" to "had" to maintain the past perfect tense appropriate for the context.

5. Each of the proposals **was** carefully considered before a final decision was made.
=> **Correction:** Change "were" to "was" since "each" is singular.

Here is a summary of the corrected sentences:

1. The board of directors **is** planning to implement new strategies next quarter.
2. If they **had** known about the changes earlier, they could have prepared better.
3. The report was submitted after it had **been** reviewed by the committee.
4. He claimed that he **had** completed the project by the end of last week.
5. Each of the proposals **was** carefully considered before a final decision was made.