Unsplitting Infinitives

Hi. I have noticed that Grammarly always recommends me unsplit the infinitive in the sentences like:

This method will allow you to instantly earn some money.

To be able to do that, you have to promptly react to the situation.

All that will allow you to properly organize your business.

Isn't that a not particularly good advice from Grammarly?

A somewhat related question, although it is not about infinitives: I have watched the Poirot series and noticed that the Belgian detective systematically uses grammatical constructions that look as follows:

It was not a suicide but murder — a murder most cruel.

All of us became witnesses of those events — the events most sinister.

Are they grammatical or just imitate the word order in French? Thanks.