Assalaamu alaykum.
1. If the bride's father is alive but does not wish to attend the wedding, can her brother be her Wali (legal guardian)?
2. Do Ijaab and Qabool (offer and acceptance) have to be said in the past tense e.g. 'I have accepted'? If they are not said in the past tense, is the marriage nonetheless valid?
May Allah reward you.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

The bride's brother cannot be her Wali in the marriage contract if her father is alive unless he (the father) delegates her brother to do so. The bride's father is worthier of being her Wali as long as he is eligible to have guardianship over his daughter. Please refer to fatwa 83338 about the sequence of the woman's guardians in marriage.

The Hanbali book Zaad Al-Mustaqniʻ reads, "The eligibility requirements of the bride's Wali are: being competent for religious assignment and being male, free, and mature."

Trustworthiness is not a requirement according to the most likely correct view of scholars. This is as far as the first question is concerned.

As for the second question, the answer is that scholars unanimously agreed that the marriage contract is valid in the past tense. They argued that using the past tense is more indicative of certainty. As for using the present and imperative tenses, they held different detailed opinions regarding the validity of the marriage if conducted with them; some held that such a contract is valid while others held that it is invalid.

There is no doubt that it is better to say Eejaab (proposal) and Qabool (acceptance) in the marriage contract in the past tense in order to avoid a situation which scholars held different opinions about.

Allah knows best.