Some inputs based on my reno experience...
1) From BTU to wireless router - there's only one LAN cable connecting these 2 equipments, not 2 cables from your diagram (refer to point 2).
2) The wireless router have 5 ports, one WLAN port (to plug-in incoming LAN cable from the BTU), and 4 switch ports. Of these 4 switch ports, only 3 can be used for data, as port no 4 is dedicated to your IPTV.
3) Yes, you can have a simple splitter plugged into your BTU RJ-11 port to have 2 DECT phones connected. I have a 3 way splitter for my DECT phone. Just make sure your RJ-11 phone lines are long enough to reach your BTU unit.
4) Other setup seems to be ok, although I am not sure if your switch on the ground floor will be compatible with the wireless router or not (I am not sure abt this, u gotta ask some sifoo).
5) I used a separate switch to handle most of the traffic inside the house, leaving most of the ports in the TM's wireless router unused (only use port 1 for internet traffic, and port 4 for IPTV). I would suggest that you do the same if you stream lots of videos or audio around your house, as the router provided by TM isn't exactly enterprise quality level

Hope this helps!

Just to be clear (since your diagram wasn't clear), Port 1 to 3 is on the unifi router are data ports, while port 4 is the IPTV dedicated VLAN port.
So Yes, Port 1 of my switch (Netgear) is connected to Port 1 of the Unifi router. The LAN ports from all around the house ends up connected to the netgear switch and does not connect directly to the unifi router.
Note : Port 4 of the unifi router does NOT connect to my netgear switch. It goes straight from the unifi router to the IPTV box, as it is a dedicated VLAN port. It will NOT work if it goes thru a switch.

I didn't utilize port 2 and 3, thus I am not sure if it will work or not, if you have a separate switch. Can't remember if I did any settings to them too..

No this will not work. Port 4 from your unifi router only works for IPTV and IPTV only. IPTV will not work if you route it thru a switch, unless you can set a VLAN port on your switch (there was a tutorial on this somewhere in LYN, do a search). This means that port will be dedicated solely for IPTV and will not share data between all other ports.

Like what weikee mentioned, there are such DECT phones in the market which allows multiple handsets for one connected base station. But I needed to connect my home alarm to the phone-line, hence the splitter has been used to cater for that. You'd need to take note of this if you plan to use your home alarm call-out feature.
Cheers!