Could I get a crash course in how Dem convention delegates are chosen and elected?

This is the first Dem primary campaign I've helped on, and I've come to realize how much I don't know, especially about convention delegates.

I've learned that each delegate, representing his or her congressional district must obtain 250 verifiable Dem. district signatures, each on a separate petition based on county of residence. Does not this favor those in urban/suburban, heavily D districts, and handicap those in rural, gerry-mandered red districts?

What happens if the delegate fails to obtain and submit enough verified signatures? Is a space left open on the primary ballot to write-in the delegate?

If voters can write-in delegates on the primary ballot, what are the rules for advertising that possibility to Dem voters in the district?

Are local, county Dem organizations allowed to support one candidate over the other? I know they help circulate delegate petitions, and I'm wondering if the rules say they must circulate them all. I assume they must.

Finally, who makes these rules? Are they agreed on by the PA State Dem. organization, or the DNC?

I'm trying to help a friend, but frankly, I never was taught this aspect of elections in my old high school civics class (way back when).

Thanks, PA Group!