1. The question is not about the legality while are getting the job done illegally.

If this can be allowed legally why should you grease the palms of the officials.

In any case you may not face any problem at least till this official is holding this post.

You may verify from the master plan of the building in which all such things would be mentioned

3. It depends. If you have the roof rights or the terrace rights on the entire third floor, you may not need to get NOC from other flat owners.

4. It is always better.

1) delhi Govt has relaxed norms for construction of 3rd floor . you dont need NOC from other flat owners for construction of 3rd floors iin Delhi . however aspects such as structural safety , fire safety , non obstruction of passages , air and sunshine of neighbors will have to be considered while sanctioning building plans . decision will be taken on merits but existing flat owners will have chance to furnish their views

2) as per Master Plain Delhi 20 21 in residential areas permission was granted for construction of 3rd floor but SC had stayed order pending final decision on master plan 21 . owners can carry out construction provided they give affidavit that they will comply with final orders of court on MPD 2021

3) advisable to use an architect

The Municipal Rules differs from state to state , Municipality to Municipality. So I can not comment whether map can be passed on the basis of affidaivt or not or other formalities as mentioned by your query is required or not.

The best option is to visit the Municipality and meet the concerned official. It alone can guide properly.This query is not for the advocate but for Municipal officials.