i do hope you were kidding here. The ember flys best in 0 (spelled zero ) winds, but can be flown in 5-7 mph winds if you don't mind hovering in one spot, or letting the wind have a large say in where the plane is going.
Foam safe ca is ok, but if the rudder has not broken clear of the tail plane, you can use thin strips of regular scotch cellophane tape. Others have used gorilla glue, pre-wetting the parts before lighlty brushing or applying the adhesive. A complete spare tail plane (rudder, fin, horiz. Stab) is available from horizon hobbies. You would need to carefully cut away all the broken parts, including the black adhesive used with the oem parts.
I haven't as yet, not needed it. If you do use cf rod or strips, be sure to use the lightest material possible. Weight is a definite no-no for micro planes such as the ember.
Not sure the exact type of foam being used, it has a thin skin on it. The spare rudder, fin, horizontal stab are around 0.078", with the fin being about 0.090 at it's thickest. Note that most, if not all of the tail plane surfaces, are shaped slightly to form a symetrical air foil, one of the reasons the ember flys so well, imho. So, if you tried to make these parts out of a sheet of foam, you might want to shape them somewhat similar to what the original parts indicate.
Order spares from horizon hobby or your lhs, use medium to thick foam safe ca, or gorilla glue.
No need for dual rates or exponential. The plane flys fine on stock throws, you can get more deflection by moving the control rods to a hole in the control horns closer to the respective surface, i.e. Rudder, elevator. I use the closest hole to the control horn for rudder, and three holes down from the top of the control horn for elevator. I can get tighter inside loops with this arrangment and a passing snap roll!
Don't mess with the stock configuration. It serves the ember quite well. Some have had good luck with the prop from the su-26 um sukhoi, but i believe this was in conjunction with some type of brushless motor. I have three embers, and the stock prop arrangement has served all of them well.
Motor temperature is really not an issue, from what i have experienced, when using proper range lipos, such as the 70, 110, 120, 130, 138, 150 mah 1s lipos. Haven't tried the 240 mah 1s yet, it might be a tad heavy. I have yet to experience a notably warm motor after a flight, and most of my ember flights are in the 15-20 minute range, the longest being 22 minutes, outdoors.
You shouldn't have a problem with wot applications, but you will get longer flights if you back off the throttle a little bit. I start (via hand launch) each flight at full throttle, then back off once i get up to altitude. I fly my embers in-doors on occassion and that calls for reduced throttle settings. However, 90% of my ember flying is outdoors, at higher altitudes and distances than what might normally be flown. Two of my embers have navigational lights, very helpful when flying at dusk, some 250 feet up and a house or two away!
As noted, the ember motor will accommodate a wide range of 1s battery capacities, easily up to the e-flite 150 mah cells. Haven't tried the 240 mah cell yet. With throttle management and warmer weather, you should get a solid 20-23 minutes on the 150's, close to that with the parkzone 120's which are very good. The 138's are probably the flightmax zippies from china. Grossly over-rated capacity-wise, more like 110 mah, but they work well with the ember/vapor, and for $2, that's a deal. The 138 zippies are absolute
For the um p-51 mustang, and probably would be less than stellar with the recent um j-3 cub. I have had great flights with the orignal 70 mah 1s lipos, with 12-15 minute flights rather common. Note that hobby king is now supplying the zippy 138 mah v2 lipos, but i have not had any experience with them yet.
The 70, 110, 120, 130, & 138 lipos are all the same length, but get thicker as you go up in mah's. The e-flite 150 is longer than these others, but can be fitted to the ember. The issue is, as you have noted, weight. The 240 cell is almost square and not designed to fit the battery clip on the ember.
For me, 22-23 minutes has been the longest flights, and that has been with throttle management and flying at higher altitudes. I get well up over the tree line to catch the smooth air flows. Neat to see the ember turn down wind, after a somewhat laborius flight upwind, then go like stink down wind, with the throttle almost off.
The more common challange has been to make the ember fly slower, via air dams, flaps, speed brakes, etc. Some excellent work done in this area, go back and review the thread for photos and videos. Increasing the speed of the ember, imho, would be counter productive. The design features that provide it's excellent flying capabilities would become detriments, once extra speed was (if possible) obtained, such as flutter in the tail surfaces, notably rudder. Enjoy ember for what it is, a great slow flying micro plane which definitely flies more like a plane than does the vapor, and that's no criticism of the vapor.
That's what we are here for.