'Lord Rama is parabrahma". That is absolutely true. Parabrahma means ultimate/superior to L. Brahma. But superior to what in Brahma. It is not shakthi (physical strength) but knowledge (mental strength) b cos L. Brahma is more for knowledge/sristi than for strength. It doesnt mean, L. Brahma is powerless or has lesser shakthi, b cos he is one among d the 3 murthis. Ulimate strength (both physical and mental) of the 3 worlds is the cumulative strengths of L.Vishnu, L. Shiva and L. Brahma (as their astras).
"LORD RAMA is SARVOTAMA". I m not pretty sure of the context. LORD HARI/VISHNU is SARVOTAMA. And LORD HANUMAN is JEEVOTAMA. There is a lot of difference between Lord Vishnu and Rama. But I do realise that devottess consider Rama and L.Vishnu to be the same and may b acceptable.
Though Vishnu is an avathar of Rama, Vishnu completely is not Rama. Rama is Vishnu by heart, mind and soul, Bharatha by chakra, Satrugna by another weapon of vishnu (not sure). Lakshmana doesnt come here b cos he is the Naag (devottee), right on which Vishnu takes rest. Meaning: he has the knowledge superior to brahma but devoid of the real shakthi's of Vishnu.
Hidden meaning: No body is born with everything, Rama is born as a Parabrahma and very well knows this. He very well knew that everything cannot be learned and everything cannot be obtained and if one has the max., victory/success is only to persons who also seeks help from other people to hold dharma. He does seek help from Lakshmana, Hanuman and lot of Vanars in a lot of circumstances.
Mentally, Rama is more powerful than Valli and by parabrahma knowledge he knows that vali is undefeatable in wars and hence he has to use a special astra to kill Vali.
"Boons of Vali given by Brahma wont affect Lord Rama as Rama himself is Parabrahma". I m not sure of this context as well.
If boons of brahma doesnt work against Rama, then if u can recall, Indrajiths brahmastra should have not work on Rama. But, Rama and Lakshmana faints against brahmastra but doesnt die. This too has lot of significance.
The significance of Bramastra is to destroy the opponent completely whomsoever it is. But, if Rama is destroyed, then the creator of the 3 world is destroyed. Meaning: If the creator is destroyed, all the 3 worlds r destroyed and brahma himself is destroyed and there would be nothing left out. Laksmana also doesnt die, b cos, according to Lord Vishnu, he has not left his true devotees and/or innocent people die. (This is wat happens in Mahabharath when ashvatama hits brahmastra at Arjunas grandson).
I had no intentions of degrading the powers of Rama. Eventhough, he is very skilled in archeries & gained lot of astras and is supreme in knowledge, against certain persons like Vali, he cant fight directly, due to the precious boons. Boons are to be respected as they r given by Great Peoples for Great Works. Literally, I dont think there is any sin.
Sorry for my big replies. I really dont know to xplain in few words as each and everything inch of God's act is a big ramayana and mahabharath.