IS there halachic basis for using other names of Hashem (instead of Ado..) in davenning and brachos, if one chooses?
Yes, there is plenty. The Shulchan Aruch rules (OC 187:1) that if one says רחמנא (the merciful One) as God's name in bentching he has fulfilled his obligation. Later, in OC 214 when discussing the requirement to have God's name in a blessing, the Mishna Berura notes that any of God's 7 names which cannot be erased (see Rambam Yesodei haTorah 6:2) are certainly no worse than רחמנא and would work as well. All this is of course to some extent bedieved as we try not to adjust the language of our fixed prayers unnecessarily.