First, this is exactly what Romesh Bhandari did in UP, to much hue and cry. Declarations of support are mere statements of intent. The proof of the pudding is in eating it, in this case in the actual voting on the Parliament floor. An individual, whether it is the President or governor, can make the wrong decision based on false declarations of support. The majority front wasn't obvious in 1996 and it isn't today.
New situations call for new solutions.
Second, much of what is 'constitutional' is really what has been practised. But we do not have an established practice for 'selecting' the executive from a hung Parliament. It is obvious to most observers that with the plurality of interests the Lok Sabha now represents, it is unlikely that any election in the near future will throw up a clear winner. The constitutional practice of inviting the leader of the largest party assumes that the largest party will have a simple majority. The assumption is wrong.