Hi, Julius. Great questions. Here's my shot at answering them.
maxime plebi acceptus erat : Why is plebi in dative? Would the phrase "Sum maxime acceptus Romanis" be acceptable?
A little editorializing first. I think it is significant that you said "I understand what it all means but the construction gives me trouble." To me, that is the right way to read. First and most importantly, you have figured out what the sentence clearly has to mean. Second, you have a question about the construction. I think there's two ways to answer the question. The first is, you have already answered your own question by having figured out what the sentence and phrase clearly has to mean. Clearly
acceptus [dative] was a common Roman phrase, its meaning clear from the context. And therefore, yes, I would think
sum maxime acceptus Romanis would be acceptable. The second way of answering the question would be to consult one or more grammar books and determine which named use of the dative you think
acceptus [dative] most clearly corresponds to - an exercise I leave to you or to another Textkitter!
eodem usi consilio : If I were to rephrase everything as Persuadeo eis ut eodem usi consilio una mecum proficiscantur would the usi still be grammatically correct? What if it were singular Persuadeo Marco ut eodem usus consilio una mecum proficiscatur?
It seems to me you understand the construction perfectly.
Utor is a deponent verb that governs the ablative (and is extremely common). Its participle is
usus. The nominative (in this case) participle must agree in gender and number with its subject.
eis :
usi (gender clearly masculine),
Marco :
usus.
qua minima altitudo fluminis erat : Ørberg explains that qua is adverbial and synonymous with ubi. Is it common? Is there any stylistic/grammatical reason why Caesar uses it instead of ubi? Or why not use the ablative quo?
Whatever Ørberg says is good for me. Is
qua common? Enough in my own reading at least that I wouldn't have questioned it. Why did your namesake use
qua instead of the synonymous
ubi? Beats me. Sounds like a project for an ambitious graduate student: Hit the concordances and write a brilliant paper on the respective uses of
qua and
ubi by time, author, and genre! Could he have used
quo instead? To me at least (and I'm too lazy at the moment to consult the dictionary),
quo implies motion toward.
Qua = where,
quo = whither. Others may wish to comment.
nihil esse reliqui : Why not reliquum?
Simply because - if the manuscripts are consistent on this point - he opted here for the common idiom using the (partitive?) genitive. I wouldn't bat an eye at either construction, though you've got me wondering if
nihil reliqui is a bit literary and less likely in normal conversational Latin?
quod debeant / quod praestare debeant : I wonder if anyone has an idea of why the editor made this (in my eyes so minimal) change.
Welcome to the reality of ancient texts and their many manuscript variants and suggested emendations. The answer should be in the critical apparatus of your Teubner edition, which I do not have at hand. Are there manuscript variants according to the apparatus, from which the editor had to choose? If so, read his preface in order to understand which manuscripts he gives the most weight to and why. Or, is
praestare an emendation? If so, by this editor or a previous one? Is a reason indicated or source given (e.g., a journal article) that you can evaluate or further research if interested? (You've graduated from Ørberg to the Teubner. If you're curious or interested in some good introductions to textual criticism, I can suggest some, but not currently since I'm out of town and away from my library.)
reperiebat ..., quod proelium equestre adversum paucis ante diebus esset factum, initium eius fugae factum ab Dumnorige : The quod gives me trouble.
I think
quod here means "because" (quite common): "Caesar came to know by inquiring that it was because [according to his informants, which is why
esset factum is in the subjunctive] the equestrian battle of a few days ago had turned out bad that the flight of Dumnorix and his cavalry was initiated."
I'm a mere amateur with no relationship whatsoever with classical scholarship.
Why not? You're clearly very good at it! Again, great questions.