First rule of SDR: It's not a scanner, and no matter what you do with whatever software you choose it's never going to be exactly like (or even really close to) a real physical scanner. At least that's what I tell people that are new to using SDR hardware and software of various kinds.
Is it difficult to do some of the things the OP is asking to do? Well, as someone that's fairly experienced using SDR hardware and software I'd say no, not for someone like myself it isn't but for someone brand new to this way of doing things it can seem to be practically impossible at first so that's why the #1 recommendation I'd make is read, read, read some more, then read even more, and when you think you're done reading you're gonna have to realize no you're not done and you're going to have to read even more than you've read up to that point.
The information is out there, really, and yes there's a lot of info that is "outdated" but it's really not because the basic concepts of SDR always remain the same: a hardware radio tuner controlled and utilized by software typically running on a computer with an operating system to run that software on.
The most common SDR software in use today is arguably SDR# and by default it won't "scan" anything as it's designed as a piece of software to control and make use of an SDR (literally meaning software definer radio/receiver/transmitter/transceiver/etc). There is a plugin for SDR# known as the Frequency Manager Suite which allows it to scan in a more traditional sense but you are still
extremely limited in terms of functionality, the most glaring one being that it's for conventional single-channel use only which means it won't work with trunked systems at all because it can't follow the trunking control channels because it doesn't work as a CC decoder for the necessary info. If you have a bunch of frequencies it can hop them one after the other, it can stop on one that's got an active transmission, it can hold on the frequency when the active transmission stops per a delay time you set, and then resume the scanning of the programmed frequencies/memories but that's about it.
It offers other features as well for logging/data analysis and such aspects but again it won't turn a computer with an SDR device into a proper scanner like one you can buy from Uniden or Whistler these days. Right now the limitation is the software that's available, and because SDR is moving in rapidly in terms of popularity the amount of actual software development isn't really very "happening" so far. SDR really exploded in popularity just over the past 3-4 years because of the discovery that those "cheap USB TV tuners" designed for the old European digital TV system could be used with modified drivers to allow them to become wideband receivers.
The concept of SDR itself has been around for several decades so it's not actually new in that sense, just new in the sense that it's become more cost-effective for a huge number of people because of those "cheap USB TV tuners" that cost from ~$10-25 that can be coupled with the software that actually is available (some of it created specifically for those types of low-cost hardware devices) and boom, now everybody is beginning to realize how useful it actually is.
With respect to the particulars the OP asked about I'd say this:
- Multiple agencies: if they're conventional you can use SDR# along with that plugin I mentioned above, and SDR# does support remote control over TCP/IP connections IIRC so you can make use of that as required but if it's trunked systems (or even one involved instead of multiples) you're going to be stuck and have to find something else, perhaps FMP/DSD+ as well if the system(s) are using digital modes like P25/DMR/NXDN/etc
- Broadcasting a feed: if you have an actual physical scanner that will still be the best source for the feed audio itself and getting that out is easy in comparison to trying to get a proper SDR setup that will scan and then output a feed to get to some site like Broadcastify.com or wherever (or just keep it private for your own use). Possible? Yes. Easy? Hardly, but still possible even so.
- Remote control: yes that too is possible but again it's going to be limited in capability if you mean direct control of just the SDR app/program/software itself - in many respects using a tool like Remote Desktop (Windows), VNC (other platforms), and so on would be easier as they'd allow you to see the "desktop" of the computer running the SDR software you're hoping to control.
I'd say if you truly are new to SDR spend some time, a few weeks or so, reading, researching, trying different things, trying different software, and a lot more reading - seriously, hands on is fantastic but without the instructions you have to read with comprehension most people won't be able to get things going and if they do it could be a one-shot and they may not be able to repeat it if necessary and of course repetition is the mother of all skill.
Also, as we have a subforum here at RR dedicated to SDR topics you might want to read a lot of the stuff there whether it seems to appeal to you based on the thread topic or not, the info there is invaluable on many many levels:
Software Defined Radio - The RadioReference.com Forums
Good luck...