Now what are these accents for :O?

Prṓpria quæ maribūs tribū́ntūr mā́scula dī́cās:
ūt sūnt Dīvṓrūm; Mārs, Bā́cchus, Apṓllo: vivṓrum;
ūt, Cáto, Vīrgiliūs: fluviṓr(um); ūt Tī́brus, Orṓntēs:
mḗnsi(um); ut, Octṓbēr: vēntōr(um); ūt, Lībs, Nótus, Aū́ster.

— uu / — uu / — uu / — — / — uu / — —
—— / —— / —— / — uu / — uu / — u (?)
— uu / — uu / — uu/ — —/ — uu / — —
— uu / —— / ——/ ——/ — uu / — u (?)

I’ve gotten that the macrons ‘ere mark metric length rather than vowel quantity, but what’s the point of the accents? Also, can a hexametre line end in /— u/ ?

Accentus vim vocabulorum denotant (per quos vox acutè elevatur; acutus accentus est).
The accents mark the word stress (when the voice is raised with an acute)

It can. Lines can end either with a dactyl or trochee, that is last, the syllable is either short or long (anceps)
Licet. Aut dactylo aut trochaeo versus terminatur (id est, anceps syllaba ultima)

Post scriptum

I just noticed that all the syllables ending lines are long in your example! -ās -um -ēs -(s)ter (long by nature and long by position)
Modò animadverti, tuo in exemplo omnes syllabae versum terminantes longae sunt (positione et naturâ).

Exerpti sunt hi versus e libro Latinae Grammaticae Rudimenta nomine apud Lilium magistrum ab Erasmo edito.

Lines from Lily’s Rudimenta (edited by Erasmus)

Yep, it’s Lily allright. I was planning on memorizing the poems of his — though I think it will have to be in that, of most purist Latinists so hated, very chop-choppy way of reading it.. Though I think that As in præsenti will give more than Propria ad maribus.

So.. Anyone has got any idea how I should go about to read this aloud? With my limited knowlege of traditional latin scansion aloud I came up with somethign like this:

PROpria quæmari bustri BUNtur MAScula DIcas

If you want to pronounce it according to the scansion technique commonly taught in schools, you would simply stress the first syllable of each foot (the ictus), as per the scansion diagram in your first post:
Própria quǽ maribús tribuúntur máscula dícas
út sunt Dívorúm Mars Bácchus Apóllo vivórum
etc.

If you want to pronounce it like an ancient Roman, you would simply stress the words as if they were prose. This is what the stress marks in your text seems to indicate, although not all of the words has the stress marked:
Própria quae máribus tribuúntur máscula dícas
ut sunt Divórum Márs Bácchus Apóllo vivórum
etc.

In the later case, you should take care not to lengthen stressed short syllables (like in máribus).

Which is how I learned and sounded odd to me even as a schoolboy. It probably put my understanding of meter back by many years.

I had to listen to Stephen Daitz (on a recording) work through this to understand what this was. Some of his steps are awful to listen to, but it made sense.

Absolutely. There is beauty in the sound of tensions and resolutions between word-stress and ictus, as you know and practice.

Indicant. Bellum sonum dissonantiae consensûsque inter vim et ictum, ut tu benè scis recitasque!.

Hoc dicere oblitus sum. Desunt, nota, haec signa in editionibus libri pristinis, et variat inter sequentes quomodo ea denotentur, secundum redactoris genium.

I forgot to say this. These signs aren’t in the first editions of the book, and can vary, anyway, in subsequent editions depending on the editor’s preferences.