I suppose 'death' in this reading is to be regarded as the one cloud which the radiation of the Countess cannot dispel. There is no indication, however, that this is the thought in Donne's mind. As punctuated (i.e. with a comma after 'subdue', which I have strengthened to a semicolon), 'But one' goes with what follows, and refers to God: 'Excepting God only, you are the most illuminating object we can contemplate.'
Page 219, l. 27. May in your through-shine front your hearts thoughts see. All the MSS. agree in reading 'your hearts thoughts', which is obviously correct. N, O'F, and TCD give the line otherwise exactly as in the editions. B drops the 'shine' after 'through'; and S96 reads:
May in you, through your face, your hearts thoughts see.
Donne has used 'through-shine' already in 'A Valediction: of my name in the window':
'Tis much that glasse should bee
As all confessing, and through-shine as I,
'Tis more that it shewes thee to thee,
And cleare reflects thee to thine eye.
But all such rules, loves magique can undoe,
Here you see mee, and I am you.