I don't think fixed line spacing will make any difference to the original problem since the spacing is calculated from the bottom of the first line of text which, because of the accent, will move. This can easily be checked by putting some sample text in a frame, then adding a first line consisting of a lower-case letter then adding an upper-case letter. Adding the upper-case letter causes the lower lines to drop down since it's higher than the lower-case letter causing the bottom of the line to drop.
SirPL, I'm just wondering: Why are you trying to align the baseline grid with the margin? Generally, the baseline grid should go where you want the text to go and shouldn't really have much to do with the margins. You might be trying to do something that you don't need to do.
It makes sense that, if the size of the baseline grid stays the same and you change the size of the text then, the visible gap between the lines of text will change. Just change the size of the baseline grid spacing to suit the new size of the text.
(Off Topic) Sorry, I don't think it's possible to assign a style to a shortcut key.