Real Estate / Kodeksu cywilnego Umowa najmu for tenants in Poland and laws governing tenancy agreements [16]

I's a bit complicated - Civil Code articles 659 - 680 regulate tenancy of not real-estate property (e.g. cars etc.), articles 680 - 692 regulate the tenancy of real estate both by professional units and consuments, and the separate Act on the protection of the rights of tenants (Ustawa o ochronie praw lokatorów) regulates the tenancy of real estate by consuments (i.e. not professionals). The rights and duties of tenants and landlords are regulated in section 2 of the Act.

When you are consument, your rights are rather well protected, since the provisions which are not in accordance with Act are not valid, so even if you sign it it will not be in force. In fact you should only pay attention if the contract is signed for the indefinite or definite period of time (if for definite, the contract lasts for time specified but you cannot terminate it earlier - its your choice), and to look out if there are no contractual penalties (kara umowna). And when terminating contract be sure to do it writing, and be careful not to get shafted on your security deposit (kaucja) - you may want to negotiate payment of last rent by the deposit.

Real Estate / Kodeksu cywilnego Umowa najmu for tenants in Poland and laws governing tenancy agreements [16]

Well, they are quite harsh, so your legal person is right, but maybe you could negotiate some provisions.
Regarding your clauses, my opinion is:
1. Interests are acceptable - the right to demand interest from any due money obligation is regulated in Civil Code.
2. 300 zł of repair costs seems harsh, but due to the Act of protection of tenants rights, the tenant is responsible for inside, smaller repairs. The parties can change it in contract, so its negotiable, but demanding payment for smaller repairs isn't exactly unlawful.

3. This is not in accordance with Act of protection of tenants rights (art 11), so its not valid anyway (you have to be due with rent for 3 months for the owner to be able to terminate contract).

4. This also doesn't seem right, normally the owner cannot enter the property without your permission. Maybe if you agree to this clause, it is valid (it is not forbidden by law), but i would negotiate it, it is not widely accepted clause. Why do they want to inspect you anyway, inspection should be done when you return the local.

5. Due to the civil code (art. 675) you are not responsible for normal wear & tear. But due to the act of protection of tenants rights you should "renew" the apartment upon return - it doesn't precise what that means, i would argue that if nothing is broken you are not responsible for any additional improvements. So i would opt to regulate this in contract as it is in 675 of code civil - return local in good state, but tenant not responsible for normal wear & tear.

There you have, free legal advice :) (Be aware i'm a law student so it may not be 100% correct).

Real Estate / Land dilemma - registering land and properties in Poland [8]

If there is no written will, by polish law all children of the deceased inherit in equal parts. If the aunt doesn't want to show the written will I suggest you should hire polish lawyer, as sorting this out will require court procedure called "confirmation of inheritance" (stwierdzenie nabycia spadku). In this proceedings the court will confirm who is/are the heir/s, and if there is a written will, your aunt will be required to present it. The good thing is that without this procedure none of the heirs can be stated in land register as the owner (unless someone was given the land before grandmothers death). The land register is available on the internet, but you have to know the number of the real estate (KW (księga wieczysta) number). You can check the KW number in local office, but they will give this information only if prove your legal interest, which means you have to prove you are the children of the deceased owner of the land, for which you will need your birth certificate and grandmothers death certificate, which you aquire from the same office (for a small fee), but you need to have it before you ask for the real estate number. So as you can see it requires some paperwork, so i think it's best if you hire lawyer with practice located in the city /town near the real estate, providing he speaks english.