Silversurfer.

You may be right on many items in your post. This is common knowledge, and it does not harm to stress the importance of these causes for interference every once in a while.

Having said that, I find it unwise to even suggest the shortening of distance between ignition-bound components, and plane controlling components.
Whilst some makes of RC equipment may be less prone to Radio Interference than others, one may get away with using other brands for a period of time like another poster suggested. The receiver brand however was not subject of this topic. The subject is ignition generated RI in general, and safe practice to prevent it.
In this practice, I would stick to common known measures to prevent RI by ignition systems, without even differentiating between modern electronic ignitions and magnetos. A few main rules should be considered here, and from these rules, safe practice results.

Rule 1
every steep angled voltage transition is built up of many components, including radio frequency. These RF components, due to their nature, can be transmitted without the presence of any conductor.

rule 2
current conductors not only convey signals by direct current transfer, but also can act as aerials, to receive, or radiate Radio interference.

rule 3
Radiated (airborne) Interference's field strength deminishes rapidly with increasing distance, typically 6dB for each distance doubling. (6dB equals about 1/4)

So in order to eliminate, or avoid Radio Interference one should:
- Shield the interference source to the best ability (all rules), and ground the shield well.
- not use current conductors (metal pushrods or wires) leading from the interference source to the radio equipment. (rules 1) and 2) )
- provide maximum distance between interference source and Radio equipment (rule 3)
- prevent, that any RI radiated into servo or battery leads (rule 2) will reach the radio receiver by using chokes or twisted cables.