Sparkly/snuggly the crow equivalent trader in Bloodborne

Firing your gun while using a two-handed trick weapon isn't typically possible, but there is at least one weapon that allows you to do this:

The Rifle Spear has a built-in gun function while in its two-handed mode. I believe that you shoot it just like a regular gun, with L2. The item description includes the line "it is the only trick weapon with an attached gun," so it's possible that it's the only weapon in the game that can do such a thing.

There are also a number of trick weapons that still only require one hand even in their transformed mode, such as the Saw Cleaver, Saw Spear, and Threaded Cane.

The summoning range has since been updated via patches. As of patch 1.05, research found here indicates the following rules:

Let x be your level.

For upper limit of level you can summon:

x + 20% + 20

For lower limit of level you can summon:

x - 20% - 20

For upper limit of level you can be invaded by:

x + 10% + 10

For lower limit of level you can be invaded by:

x - 20% - 20

It should also be noted that if you are using passwords to summon a friend, this limit will be ignored and you can summon them regardless of their level. Players who are significantly higher than the summoner who are summoned this way will have their levels scaled down.

Here is a nifty calculator which does the math for you!