Whether there are those within the pro-Russian element with the expertise to operate such a missile system or not is open to conjecture. To my mind it is not impossible that those who had served (or may have been serving up to recently) in the Ukraine forces to now be opposed to them. One of the main points of resolution in this is whether such systems as the Buk 11 were/are present in the areas in control of the forces in question. Additionally the actual version of system is equally pertinent. Both sides of this Ukraine argument have much to lose or gain depending on the outcome of the investigation. If Russia supplied the system directly to the pro-Russian element then it will suffer an even greater penalty regardless of its intentions.
From the news and pictures I have seen it would appear the crash site has not been secured. In any accident it is imperative to secure such an area to prevent loss of evidence, looting/theft and, sadly, tampering with bodies. It would appear that those with any level of control in the area are either totally unwilling to cordon the area or have no comprehension of the needs. From the little I have seen it would appear that some luggage has been deliberately opened and from this I can only presume that personal belongings and other significant material may well have been removed.
I can fully understand that where bodies and body parts are in exposed positions (on roofs, in gardens and similar) they need to be secured to a safe facility. This action is normally taken by competent persons who would label the body/part with its location and other data for later identification. In general no other disturbance of the site would be made until a team covered the ground recording and making such recovery as necessary. These tasks are far from pleasant but absolutely essential to gain an accurate data for analysis of the cause and effect.