OK, there's clearly an occasional short circuit somewhere. I don't have a
Subaru, but offer the following suggestions.

The fuse is listed as E/G IGN SRS. This suggests to me that the fuse
protects  three items: E/G, IGN and SRS (Airbag system).  IGN is clear
enough, as is SRS. I don't know what E/G stands for: do you? (Engine,
Environment, Entertainment, G???)

Anyway, IGN will be passing power to the ECU most likely through a relay or
two, and also to the SRS controller. Since this fuse blowing is getting
worse, the short is intermittant. Electrolytic caps in controller boxes are
a source of unexplained sudden shorts.

Most likely scenario:
Find and remove the ECU box and open the can. Have a sniff for something
that smells sort of fishy. Leaking electro caps often have the top or base
popped out, or you can see leaking whitish electrolyte. A bad cap on the ECU
would explain why the engine won't run and why you need multiple fuses. The
good news is that replacing caps is not hard, you just have to be careful in
desoldering them. You might also consider locating a spare ECU from a
wrecker. There are also people on the net who repair / exchange ECU's.

Less likely:
You could locate the SRS controller box and disconnect it. DO NOT DISCONNECT
THE AIR BAG DIRECTLY (these have yellow connectors I think) unless you want
to risk setting it off via static discharge: Just disconnect the power to
the controller box. I doubt it's the SRS system.

If you can identify what E/G means eg by pulling the fuse and seeing what
else won't work, isolate and check these items if possible, for bad grounds
or potential shorts.

Lastly, its not impossible the you have a short inside a wiring harness.
These can be really hard to find until the short generates enough heat to
melt something ...

SD