Is it permissible to agree to a new suitor with the intention of returning to the first husband, and is this intention considered a form of "tahlil"?

If a man desires marriage for the sake of marriage, not for the purpose of making a former husband lawful [for remarriage after an irrevocable divorce], then there is no harm in your acceptance of him. You are not required to inform him about the pain of intercourse. What is on your mind regarding returning to your first husband does not matter; the criterion is the validity of the contract and the husband's intention, while the woman's intention is of no consequence.