1999 subaru impreza outback sport with rust on the under carriage

its under the glove box facing up. you remove it by dropping it down.
there should be 2-4 screws and the drain hose

depends, if it just not working, or if it has restricted flow. If its restricted, get it done ASAP.

First check for an owners manual(it will say), if there isn't one a lot times there will be a sticker on the inside of the gas lid saying what to use. However, unless to vehicle is turbo or is higher compression than stock, i am quite certain 87 octane will suffice.

no adjustment, computer controlled, you're problem rests elsewhere, do a compression test,

on the top of the engine block under the inlet mannifold

I would do that plus test the plugs, wires and coil pack.

If you have any of the large chain auto parts stores in your area, call them and ask if they do free scans; most do.
If Subaru has maintained the same procedures for a while, and you have checked fluids, etc., you can try resetting the light via the ignition switch:
-Turn on the ignition (don't start) - switch it rapidly between the 'ignition on' and 'ignition off' positions three times.
If the error has cleared itself (not uncommon), the check light will reset. If the problem is still present, it will either stay or come back on.
Our Su has done this a couple of times and with the scanner we have on hand, there was never any particular repeated problem. Just a computer glitch/hiccough -

check all grounds

check ur idle speed control passenger side in front of intake i think

Each head lamp also has it's own relay, you can rule the relay out by swapping the left head light relay with the right head light relay & see if the problem moves to the other side...The relays are also in the same under hood fuse box.