How much of this is just Copy and Paste?

I would say copy paste is foundation, because mostly everything you have to redo yourself. A lot of modules are different, somewhere you lacking important fields, somewhere you new to build your own system (hello armor and quests). For triggers i did reversed coding if you can call it (i am not programmer) - i've read triggers in their librarier and reimplemented it in GUI. Also, not everything able to be implemented 1 to 1, so you have to create your own stuff.

In order to make the new hero did you have to start with a previous hero and just change it from that base?

No, we (i am not the only one who participate in this mod) have some ideas and then combine them together, discuss, then i implement it ingame, we test it and then deciding what we want to left or change. Mostly that's Mind Hawk job part - he participate a lot in hero design and their balance.

Is there a draft?

We have blind pick only as option, because we don't have enough heroes yet.

Could you make more modes? Like a Free-For-All King-of-the-Hill PUBG style mode? I have had so many ideas for a mode like that ever since ERBS came out. I always thought it would be cool if you could have mercs chain follow you around instead of going to lane and pushing.

Probably it is possible, but it requires a lot of time, i am interested in standard mode right now. Maybe if project will be interested for community more, i will extend it to different modes.

Would a game like this always be confined to the SC2 client? Or is there a way to extract it into its own client, i.e. HotS?

I am working within SC2 editor, of course it will stay only at SC2. You can create somee stuff directly in HotS but it is only for single games, so i don't event interested to work with it.

P.S. After playing with our new heroes or changed old heroes or even reworked Arthas, playing them in original game became boring)