The sages state that the first four are approved (in the case) of a Brahmana, one, the Rakshasa (rite in the case) of a Kshatriya, and the Asura (marriage in that) of a Vaisya and of a Sudra. But in these (Institutes of the sacred law) three of the five (last) are declared to be lawful and two unlawful; the Paisaka and the Asura (rites) must never be used. For Kshatriyas those before-mentioned two rites, the Gandharva and the Rakshasa, whether separate or mixed, are permitted by the sacred tradition.
So "the sages", whoever they are, apparently say that the Gandharva marriage is forbidden for everyone, but the Manu Smriti is saying that they're wrong, and that a Gandharva marriage is actually permitted for Kshatriyas.
Regarding Duryodhana, he was in heaven not because he has been
untruthful, but because it is said that if a Kshatriya dies while
fighting in a battle, he attains heaven because this is Kshatriya
dharma. He died during battle performing his duties therefore he was
ought to get heaven.
So, in general, is Hinduism a bit partial to Kshatriyas?
"Hinduism" (a dharma and it's not same as a religion) is not about any impermanent worldly systems/customs/relations e.g., the varna-system. It is ultimately about mukti i.e salvation, and before that, to lead a life of OM, by performing karma as per dharma, that rather is the most personal thing for each human being. The only things that are universal are the laws like that of karma, rebirth etc., and the natural phenomenon governed by the divine elements, or other divine beings.
So, as I said, the dharma "Hinduism" is absolutely personal, it's absurd to attach a general notion with it, like asking what you have asked. The river Ganga, never cares whether the person coming to her would worship her, or spit into her. Similarly, Hinduism doesn't care about the varna, rather it's the opposite i.e., it's according to their own varna, people tend to act.
Varna defines a state of a living "brain". Like Kshatriyas are the humans who have "dvaita state of brain", as they emphasize on the separation of their inner self from Brahmm. They realize the "link" of wisdom connecting the Brahmm and the inner self. They are most fit to be kings and leaders, because they are wise, (a quality different from intelligence) and for both vaishya and shudra, they are like links to the absolute.
This is the Kaliyuga era, and today, everyone is a shudra, with a state that just knows how to "react", sometimes to internal void, and sometimes to external lure. This state of mind is binary.
Your statement and understanding:
Kshatriya can do force sex and marry (however not encouraged)
is same as saying:
because some humans are cannibals, humans can do cannibalism (however not encouraged) OR because some people do incest, so whole humanity is fine with it (however not encouraged)
And the reference you gave takes help of the most non-trustworthy source of information about Hinduism. You should know that most resources that are on Internet are most likely to be forged/misinterpreted. No document about Hinduism on the Internet, is older than 200-300 years, and they were all of some European source. This should make you more skeptical about their genuineness, because that was the Victorian Period of aggressive Catholic movements. They were all, (more or less) evangelist in the form of historians, "scholars" of Hindu texts.
It's a pity that because of this Western site's policies, and because of the shudra thinking behind it, only "written texts" (which are mostly Victorian), are considered "authentic". I wish that someone soon starts a pure Indian site like this one.
Now, what is a life of OM? What's dharma?
Your second perception that:
Kshatriya will go to heaven if he dies fighting in battle, without the matter that he was on which side...
...is consistent, but not just with respect to kshatriya, rather with all varnas.
The idea to grasp here is that, karma is not "judged" in similar terms, as prevalent in modern context, i.e., by taking its "effect" into account. Rather it is judged only by taking its "cause" into consideration. If a karma is done because of attachment, then it's vikarma, in "simple" terms "bad karma", and vice versa. This whole prakriti, along with everything in it, is changeable, and to fall into it's moha, and in that spree, to decide "good" and "evil" is considered non-nonsensical in Hindu philosophy.
Compare a person killing hundreds, in a battle between two nations, with a murderer killing one person, for sake of his lust. Which one of them would you find "guilty"?
To "flow" unattached like a river is dharma. It is like to "live OM". Whoever, irrespective of whatever he does, if is unattached, gets salvation.
And finally, like the unbiased Mother Ganga, Hinduism also never takes sides.
Note: I seldom provide references in my posts, because I think looking up to a book every time you "think!", is again not a Hindu trait. Vedas were in the souls of ancient sages, they have not mugged it up either. Reading Vedas should be a journey of mind to soul, rather than of memory from one synapse to the other.
Mind you, that there is a difference between the Vedas and books of other religions. The "Vedas" are not "holy" as in "Holy Bible" or "Holy Quran", but they can for sure make you holy.
Let's look at the other side:
A Kshatriya must not receive alms (it is allowed for brahmins), must not turn their back in battle (even if enemy is 10x stronger), must protect anyone who surrenders to him (even at the cost of his own life) etc.
The concept of Rakshasa marriage you mentioned (taking girl away by force) is not allowed for Brahmins. However, Brahmins have the concept of kanya-daana (gifting the bride to a Vedas-trained bachelor), for which the reward is BrahmaLoka (as evidenced by mantra 'Kanya kanaka ... jigeeshaya'), which is much higher than SvargaLoka that Kshatriyas go to when they die on battlefield
Like any library, Hinduism Stack Exchange shares great information, but does not offer personalized advice, and does not take the place of seeking such advice from any Acharya, Pundit, astrologer, Guru or other trustworthy Counselor.