Not to sound like an F-22 cheerleader but I thought this was interesting. . .

You are approaching to the problem from the opposite direction,to solve the
problems you described correctly you have to install receiver/processor unit of
multistatic system inside every SAM,which is currently technologically and more
importantly financially not feasible.
But solution is very cheap,though not so excellent like turning SAM missiles
into multistatic processors.
1)Multi statics can track stealth platform at extremely long ranges.
2)Stealth platforms designed to reduce backscatter.They reduce backscatter
significantly but total elimination of bacscatter is not possible.(Thats the
reason why a particular backscatter radar detects conventional aircraft at 100
m but identical sized stealth aircraft only at 5 or 10 miles)
If your radar receiver comes close enough to stealth target (or target comes
close to bacscatter receiver) at some point backscatterer receiver will start
receiving backscatterers from target.

So,
1)You are tracking your target precisely using multistatics (You might not even
need very precise tracking using multistatics (expensive),If you use the
methods used by Serbians,you can detect stealth ,but you cannot track it.(your
SAM crews must be lighting fast)

2)If you want to use an semi active system ,turn on guidance radar and aim it
according to multistatic radar tracking data.
(or if you use serbian style interconnected bacscatterers to the latest known
position position )


Do you know what "semiactive" is/means?






3)Fire missiles guide them to target by command guidance


Command guidance? I'll bet a HARM would just LOVE that.



,as missile nears to
the target missiles own backscatter receiver will be able to receive
backscatter signals (not forward scatterers used by multistatics) from its own
guidance radar.


An active radar seeker on a AAM likely wouldn't work very well against
stealh. You'd be better off with an IIR seeker.





If you can use an active homer skip step2 and use missiles active seeker as
terminal guidance only.


A high frequency radar against a stealth aircraft?