On most installations the DME is situated around the midway point and zero'd at the touch down point.
In general although the aircraft in front will reflect the ILS signals its position means that those reflections will bounce off away from you and you won't see the effect. The same does not apply to reflections from aircraft on the ground passing close to the aerials.
The aircraft in front could in some circumstance block the ILS signal but the placing of the signal sources means that it won't normally be in the path between you and the signal source. Again aircraft on the ground are more likely to be between you and the sources.