All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Your question included three points but the answer to the first and third points may be summarized in the following:
If the matter is that the landlord rents his estate for a known period in return for a known price but he conditions on the tenant to pay the entire fee at the time of the contract, or that he pays part of it while a small fee will be paid monthly or annually, and after that the landlord invests the amount of rent and benefits from its profit, then this is permissible even if the amount of rent is small.
On the other hand, if the matter is that the landlord agrees with the tenant that the latter pays a small fee but he lends him an amount of money from which he benefits during the time of the contract and then he gives it (the loan) back to him when the tenant returns to him the rented property, then this is not permissible as this is a loan that involves benefit which is Riba (usurious interest).
However, if they agree on a known amount and the contract is concluded according to it, and later the landlord needs money and the tenant lends him some money, then this is permissible.
Nonetheless, it should be noted that there is some ambiguity in these two points which you did not clarify in the question, because asking a clear question is half of the answer.
As regard the third point about Tawarruq, then this matter is discussed in details in Fataawa 131558 and 90815, so please refer to them.
Allaah Knows best.