Ubinam gentium sumus? –
I take this to mean “what kind of a people are we?”
That’s about right.
Ubinam is an emphatic form of ubi. Lewis and Short note the genitives loci and terrae with ubi, but a genitive can be used with other words, too – “where in the world?”:
4 With terrarum, loci (v. terra and locus): non edepol nunc, ubi terrarum sim, scio, si quis roget, Plaut. Am. 1, 1, 180: quid ageres, ubi terrarum esses, Cic. Att. 5, 10, 4: ubi loci fortunae tuae sint, facile intellegis, Plaut. Capt. 5, 2, 5: ut inanis mens quaerat, ubi sit loci, Plin. 7, 24, 24, § 90.—
http://perseus.uchicago.edu/cgi-bin/philologic/getobject.pl?c.19:10.lewisandshort
Ubinam gentium sumus? – this means something like “where in the world among nations are we?” “what kind of nation are we in?” – an expression of exasperation or indignation.
dixisti paulum tibi esse etiam nunc morae, quod ego viverem – quod is a relative pronoun. Its antecedent is paulum, and you can take it as accusative of duration, Allen & Greenough sec. 423:
Time when, or within which, is expressed by the Ablative; time how long by the Accusative.
Accusative:—
diēs continuōs trīgintā, for thirty days together.
“cum trīduum iter fēcisset ” (B. G. 2.16) , when he had marched three days.
http://www.perseus.tufts.edu/hopper/text?doc=AG+423&fromdoc=Perseus%3Atext%3A1999.04.0001
The quod clause is subjunctive because it’s part of what Cat. said. Loosely, “you said that there was just a little time [paulum morae] that I would be alive.”
tibi is “ethical” dative – “for you,” but it’s hard to capture this in English. It reflects Catiline’s calculations, his erroneous belief in his own ability to control events.
Domum meam maioribus praesidiis munivi atque firmavi, exclusi eos quos tu ad me salutatum mane miseras, cum illi ipsi venissent quos ego iam multis ac summis viris ad me id temporis venturos esse praedixeram.
praesidiis is neuter. It doesn’t mean guards, but rather something like “protections”; maioribus praesidiis – “greater protections.”
iam multis ac summis viris . . . praedixeram – “I had already predicted to many very important men”. multis ac summis viris is the indirect object (or some other kind of dative) of praedixeram. multis ac summis – in English we would not use a conjunction to join these two words. “Many VIPs.” He is emphasizing that he knows Catiline’s every move in advance, and accurately told many men of the highest rank in advance what would happen.
Venissent – pluperfect because they had come before he shut them out. In English, we would say “when they came,” but Cicero’s Latin is more precise as to the sequence of events.