I won't try to argue the trinity part because I'm not sure where I stand myself on this issue. I believe it is a doctrine and can be argued either way using scriptures - at least, that's how far I've gotten in my own studies. It may be impossible for us to explain with our current understanding of living beings - for example, how could you explain to a person in 300BC that time and space can be manipulated by speed? Yet it's possible.
As for the paradise on earth - that one is fairly easy:
The old testament (Psalms) was written for the Jews prior to the Messiah's coming - at that time NOONE had the hope of a heavenly life, they only looked for the Messiah and a resurrection.
When Jesus came and was resurrected he opened the way to a heavenly hope. You will note that in the new testament there is no scripture that states that the dead are "unconscious" or anything like that - it always speaks of an afterlife.
As for the meek inheriting the earth - John answers this one at the end of Revelation. This is a scripture the JW's are very familiar with and yet seem to not really SEE - Revelation 21:3 "Now the dwelling of God is with men and he will live with them".....how will God live with men if he is in heaven and men are on earth??? Verse 2 tells us "I saw the Holy City, the new Jerusalem, coming down out of heaven from God..." - if you read the entire chapter 21 it seems clear that EVERYONE will live on earth, including God. Verse 1 tells us that this is the "new heavens and earth".
Read Luke 16:19-31 and ask your friend to explain this one - if the dead are just DEAD then was Jesus lying about this?