One of the worst maneuvers to perform against an adversary in a nose to nose engagement is a belly up turn (belly perpendicular to adversary radar) and while the widely spaced engines give the MKI an aerodynamic edge it makes it more visible to ground radar. These are some design trade offs and key difference between western and Russian design philosophy. The Russians believe aerodynamic efficiency is worth the trade off while we beleive closely spaced engines present a smaller target to ground radar and allow our planes to take more battle damage while remaining airworthy.
A few things, the F-15 will never go out on a mission in a clean configuration, it will always carry some ordanance, not to mention external fuel tanks, so whatever rcs the F-15 had (from the belly) in a clean configuration would mean nothing once a combat load is added. BTW the F-14 has been in 12 wars or skirmishes and only 4 were lost to ground fire, it should be noted that its 'belly' is very similar to the Sukhoi, infact it appears to be even wider. Also the SU-27 has been involved in 6 wars or skirmishes and only 2 were lost to ground fire.
And widely spaced engines actually give an aircraft better battle feild survivability not worse, the problem with engines in close proximity is that one hit will likely destroy two engines. This is why the A-10 has its engines so widely spaced.
Recall the Israeli F-15 that was able to RTB safely with the entire right wing missing after a collision - an MKI in contrast will be unflyable in the same condition.
You do not know that, nor are you qualified to come to that conclusion
What saved the F-15 was its speed, pilot skills, and lift from the intakes which the SU-30 would also enjoy, infact the F-15 landed at twice its regular speed; anything less and it would go into a spin. After the F-15 incident happened McDonnell Douglas concluded that the F-15 was able to maintain control because of its speed, they described the F-15 as that of a "rocket" because the pilot maintained a high airspeed, so there was nothing special about the F-15 other then the fact that it maintained a high airspeed and that its intakes provided lift.
You have also overlooked two hudge advantages of the SU-30 if it was in the same situation; firstly, because of the SU-30's widely spaced engines the pilot can independently regulate the throttles on either engine which would compensate or atleast off set any counter forces created by an unstable aircraft. Secondly, the SU-30 enjoys TVC coupled with widely spaced engines which would also be helpeful if its wing was ever lost because its engines can direct thrust in a parallel maner, or in other words, one nozzle is directed up while the other is directed down. Again this will help to compensate any counter forces created by a missing wing.
While many here don't look beyond engine thrust, experts factor engine footprint - length and diameter. Russian engines especially the ones with TVC nozzles are longer and wider. The F-414 engine is 154/35 inches while the MKI's AL-31 (excluding nozzle) is close to 200/50 inches. The MKI presents a much larger target for all the reasons I mentioned above and more.
Length and width of an engine mater little when they are internal. If size of an engine was so crucial then Pratt & Whitney as well as Lockheed Martin made a big blunder with the F-22's F-119 engines coming in at 203 inches and If the F-22 ever receives the F-35's F-135 engines which measure in at 220/51 then the F-22 is in big trouble, atleast according to you. Size matters depending on how large the difference is but what is more important is how the engines are concealed or in other words how they dissipate an IR signature, for instance the Rafale's engines (not exhaust plum) give off less IR then the Typhoons engines.
Also why on earth are you comparing the F-414 engine which is a smaller class engine found in the F-18 to the AL-31 which is a heavy class engine found in the SU-30? I though you were talking about the F-15.
The F-15 uses the F-100 engines so it would not be fair of me to compare the F-15 and its F-100 engines to, for example, the Rafales M-88s and then declare that the SU-30 has the smaller 'footprint' based on the M-88's figures, now would it?