If you’re price and complexity sensitive, I'd suggest a 9.2 channel 3-Zone Receiver and for the Porch speakers, an external amplifier with auto-on and auto-switch capabilities like the Dayton Audio APA102. It would be preferable for the Receiver to have a phone app to control Zone-2 and Zone-3, but you can use the Receiver remote otherwise. Most Receivers at this price point will have an phone app though.
Configure to use the extra 4 Receiver channels for Zone-2 and 3 speaker outputs. Zone-2 speaker outputs will not be used. Configure Zone-2 volume to be variable (not fixed). Configure the Zone-3 volume to be fixed (not variable).
Add a volume slider in the Kitchen between the Zone-3 speaker outputs and the Kitchen speakers. This allows for local control of the volume for your Kitchen speakers without having to have a remote handy. You'll use the Receiver's phone app to control Zone-3 on/off and source.
When mirroring the Den source to the Porch you'll control Zone-2 on/off, source, and amplifier volume using the Receiver's phone app.
Configure the Porch TV to turn off the TV speakers and set the Analog Audio output to variable (controlled via TV remote). If you don't have Analog Audio output for this TV, the headphone output usually works exactly this way, instead.
For easy native control for non-techies, I'd consider just getting a Roku TV for your Porch TV so you can just use the standard TV remote for control.
Sources for the Den -> Receiver HDMI inputs
Receiver HDMI 1 output -> Den TV
Receiver HDMI 2 output -> Porch TV
Receiver 5.1 Speaker Outputs -> Den Speakers
Receiver Zone 3 Speaker Output -> Kitchen Speakers
Receiver Zone 2 Line Out = Porch Amplifier INTERRUPT
Porch TV Analog/Headphone output -> Porch Amplifier MAIN
Porch Amplifier Speaker Output -> Porch Speakers
How it works:
When you want to watch TV in the Den, it's just a normal system.
When you want to add audio in the Kitchen, turn on Zone 3 and set the source using the Den remote or the Receiver's phone app (or any other control system controlling the Receiver via TCP/IP). Use the local volume slider to control the volume.
When you want to add audio to the Porch, turn on Zone 2, set the source, and control the volume using the Den remote or the Receiver's phone app (or any other control system controlling the Receiver via TCP/IP). The Porch Amplifier will automatically switch on and default to using it's MAIN input and play audio from the Receiver Zone 2 Line output.
When you want to mirror the output from the Den TV to the Porch TV, switch the Porch TV's input to HDMI and use the Receiver's phone app to turn on Zone 2, set the source, and control volume. The Porch Amplifier will automatically switch on and automatically switch to using it's INTERRUPT input and play audio from the Receiver Zone 2 line output.
When you want to watch the Porch TV independently of what's playing on the Den TV, turn on the Porch TV, use the Roku apps, and control the volume using the TV remote. The Porch Amplifier will automatically turn on and default to the MAIN line, which will play the output from the Porch TV.
1) If the someone sees the volume slider in the Kitchen fully down, they may think sliding it up will turn on audio. That will be necessary, but you may also need to turn on Zone-3 for the receiver.
2) Just because the Porch TV is off, doesn't mean that no audio will play. If Zone-2 is turned on, even if the Porch TV is off, then audio will start playing based on whatever the Zone-2 source is selected. You'll want to make sure you turn of Zone-2 before turning off the Porch TV. For example: 1) you're watching the game through a cable box on both TVs, 2) you turn off both TVs and the cable box when the game is over, 3) late that night, someone turns on cable box to watch TV in the Den, 4) the Porch speakers will start playing the cable box sound immediately, possibly waking up your neighbors.
NOTE: If you can find a receiver that forwards *all* audio signals over HDMI for Zone-2, then you can get around this by skipping attaching the Zone-2 Line Output to the Porch Amplifier INTERRUPT. But I'm not sure if any Receivers have this feature - most only output audio signals to Zone-2 over HDMI when the input is 2-channel PCM (which is rarely the case for video sources these days).
3) After you turn off Zone-2, the audio won't play from the Porch TV for a brief period of time. This is because the Porch Amplifier needs to switch back from INTERRUPTING to MAIN. This switch-back behavior is delay-based.
NOTE: You can trade the INTERRUPT and MAIN line to shift this delay period to either the Porch TV output or the Zone-2 output.
NOTE: You can get around this by just introducing a manual switch. This means you'd press a button on a switch each time you need to switch between Zone-2 and Porch TV audio (in addition to turning on/off Zone-2).
Each of these caveats may come with a WAF (wife-acceptance-factor) cost. Most other bulletproof designs will require pretty extensive control, integration and coordination components to get a really solid functioning system. This means more cost and more advanced programming and equipment. So if you can live with the above caveats, and are price and complexity sensitive, then I'd lean towards this solution.
Otherwise, get ready to dive in and do a LOT more research or just pay for a company to come in and do it for you. There's a tradeoff in solutions based on whether you care more about configuration complexity or more about cost. If you care about cost, I'd consider not trying to mirror the two TVs for video (you can still play the same game/movie/etc., it just won't be perfectly in sync), and using analog audio splitters from a single source for audio mirroring (so that whole-house audio is in sync). If you want to absolutely have video in sync between the two TVs, then you'll need to look at a matrix solution, which will add more complexity and cost to the design.
And if you can send me over $150 via Paypal for the hour I spent designing the above system for you, that would be great...
(kidding of course)