To get off this Tower of Babel, I have to go back to the basics. "What problem are you trying to solve here"? The problem is not how to make a static IP address work on your network, or understanding how DHCP operates. It's setting up your overly smart phone on your network so that other devices can find it by IP address. Normally, such things are done using a local DNS (or RADIUS) server, but those would add far too much complexity. Therefore, you use a pre-assigned or pre-allocated static DHCP IP address and you're done. If your router has this feature, I can assure you that its DHCP server will not hand out a pre-assigned IP to any other device (unless someone spoofs the MAC address and untangles the TCP and IP sequence numbers).
You remind me of one of my customers. This is an exact quote from him: "Fix it, but don't change anything". You're suggesting increasing the complexity by adding yet another layer acronyms to your problem, while demanding no changes. That's not going to happen.
Nope. I just got off the phone with a customer. "Drop everything and come over. All my files have disappeared". There went my afternoon.
Nope. Mounting a filesystem or directory structure requires a server running somewhere. SMB (SAMBA), Netbios, ftp server, TFTP, etc running on the PC. All these will require client software on the phone. Or, you could buy/build an NAS (network attached storage) server, which usually has a half dozen assorted servers built in (including ftp): Plenty of others available, some which are specialized for entertainment media, such as: which unfortunately requires additional software on both the client and server, but which might be worth the effort. As you may have noticed, I'm not a fan of using a desktop as if it were a server.