It is hard to understand from your description how your DSL line is wired. A properly installed alarm system is the first item after the NID and the phone line is routed through the alarm system so that it can interrupt any ongoing call in order to place the alarm call.
Placing a DSL microfilter in front of the alarm system would block the DSL signal from anything behind it (which would include the DSL modem!).
The only sensible way to combine the partially conflicting requirements of DSL and a secure alarm system is to install a POTS splitter between NID and alarm system. The inhouse phone wiring remains attached at the output side of the alarm system just like it is now (or at least how it should be now). A new twisted pair cable needs to be connected to the data terminal of the POTS splitter and must only be connected to the DSL modem and nothing else.
Note: none of this explains why you sometimes see 10mbps and at other times 56kbps unless someone is playing around with adding/removing filters.
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