invidiam posteritatis. Subjective genitive, 'hatred of (felt by) posterity.' fortitudinis, just below, is objective, 'unpopularity of (attaching to) firmness.'
per te cognitum, 'known by your own exertions only'; cf. [1. 11] 'per me tibi obstiti.' Cicero was a novus homo; i.e. none of his ancestors had held a curule office.
tam mature. The age which a citizen must attain before becoming a candidate for the several offices was fixed by the Lex Villia Annalis (180 b.c.). Cicero appears to have been chosen for each magistracy suo anno, i.e. as soon as he was legally eligible. He was now 43.
per omnes honorum gradus, 'through all the degrees of office.' By the Leges Annales of Sulla, the offices of quaestor, praetor, consul, had to be filled successively in the order named.
si summi viri, etc. See [notes on 1. 3], [4].
parricida, because he is attacking the 'patria' which is 'omnium nostrum communis parens.' But the word is sometimes used in a more extended sense of wilful murder or sacrilege.
redundaret. Metaphor from the overflowing of a stream: 'lest any flood of unpopularity should overwhelm me in the future.' (Cf. the English phrase 'to redound to one's credit.')
Quamquam, etc. The first 'qui' is consecutive ('tales ut'), the second is the connecting relative ('hi autem'), hence followed by indic. 'aluerunt.'