Quamvis comitia longa sint, sunt qui usque ad vesperum manere velint, ut suffragium ferant.

Because quamvis meaning although or however much normally introduces a concessive clause with the subjunctive in classical Latin.

So:

• Quamvis comitia longa sint = Although the elections/assembly may be long
• not indicative sunt

The subjunctive here does not necessarily make the idea doubtful in English. It is simply the standard grammar after quamvis in this kind of sentence.

Comitia is a neuter plural noun. It refers to a public assembly, especially for elections or voting. Even when it may sound singular in English, in Latin it is grammatically plural.

That is why:

• comitia = nominative neuter plural
• longa = nominative neuter plural adjective agreeing with comitia

So comitia longa means the elections/assembly are long.

If the noun were feminine plural, then longae would make sense, but comitia is not feminine.

This is a very common Latin idiom:

• sunt qui = literally there are people who
• more naturally: there are some who, some people ..., or there are those who ...

So:

• sunt qui usque ad vesperum manere velint = there are those who want / are willing / would be willing to remain until evening

This is not just a plain relative clause. It is a standard way in Latin to speak about an indefinite group of people.

Because after sunt qui, Latin usually uses a relative clause of characteristic.

That means the clause describes the kind of people being talked about, rather than simply identifying specific individuals.

So:

• sunt qui ... velint = there are people of the sort who want / would want ...

If Latin used the indicative, it would sound more like a straightforward factual description of particular known people. The subjunctive gives the sense of people who characteristically do this or people such as to do this.

Usque ad means all the way up to or simply until with a strong sense of duration or limit.

So:

• usque ad vesperum = until evening, right up to evening

Notes:

• ad takes the accusative
• vesperum is the accusative form here

The phrase emphasizes that they stay for a long time, not just briefly.

Because ut here introduces a purpose clause.

A purpose clause answers the question for what purpose? or why?

So:

• manere velint, ut suffragium ferant = they want to remain, in order that they may cast their vote = more natural English: they want to stay in order to vote

Latin purpose clauses use:

• ut
  • subjunctive for a positive purpose
• ne
  • subjunctive for a negative purpose

That is why ferant is subjunctive.

Yes. It is an idiomatic expression.

Literally:

• suffragium = vote
• ferre = to bear, carry, bring

But together suffragium ferre means:

• to cast a vote
• to vote

Latin often uses very concrete verbs like ferre in expressions that English translates more abstractly.

They belong to different clauses and do different jobs.

• sint is in the concessive clause after quamvis
  • Quamvis comitia longa sint
• sunt begins the main clause
  • sunt qui ...

So the structure is:

1. Although the assembly is long...
2. there are some who...

The repetition may feel a little awkward to an English speaker, but it is perfectly normal in Latin.

Yes. The easiest way to understand the logic is:

• there are some who want to stay until evening
• so that they may cast their vote

In other words, the purpose of staying is voting.

So the internal structure is roughly:

• sunt qui
  [usque ad vesperum manere velint]
  [ut suffragium ferant]

The purpose clause explains why they are willing to remain so long.

Any of those may fit depending on how literal you want to be.

• volo basically means want, wish, or be willing
• in a subjunctive relative clause like this, English often uses:
  • want
  • are willing
  • would want

So sunt qui ... velint could be understood as:

• there are some who want to stay
• there are some willing to stay
• there are those who would stay

The exact English choice depends more on style than on a major difference in Latin meaning.