SE's website price is nonsense. No one pays that.
MWBCs are certainly allowed today; the only reason they're falling out of favor is that AFCIs are challenging. Now, some panel makers have AFCI MWBC solutions for their panels. Schneider is not one of them; HOM is their el cheapo line and they under-support it to protect their QO industrial tier line. If HOM was an all-singing, all dancing line, who would pay twice as much for QO? But they have no answer in QO either.
But since it's a retrofit, instead of AFCI at the breaker you can have AFCI at the first receptacle.
Of course, the fact that it's a MWBC complicates matters. Nobody makes 2-pole AFCI receptacles, so if you're sharing the neutral beyond the first receptacle, that is not going to work.
Here, you are saved by the fact that you're not required to have AFCI at all on the existing parts of the circuit because they're "grandfathered". Therefore you do not need to comply with the retrofit rules at all (which seek to protect the wiring in the walls, that being the top reason they want AFCIs on so much stuff). It's like if you have a 1954 Dodge and you put seat belts on it, the government can't tell you what kind.
Therefore you are at liberty to simply use AFCI receptacles at each receptacle location, don't use the Load terminals at all, and call it good. You can't split a receptacle, so you'll have to put the whole receptacle on one hot or the other.
On your 240V outlets on that MWBC, simply don't put AFCI on them at all. There is no achievable way to do that short of that unobtainium breaker.
On the circuit extension, AFCI may be required to be arranged so it protects the wiring in the walls. In that case, you can extend off the Load terminals of an AFCI receptacle, but not as an MWBC. You will have to pick one hot or the other.
If you want to extend both sides of the MWBC into an addition, then blow out that last existing receptacle box into a 2-gang box, mount two AFCIs there (one on each hot wire), then extend dual /2 cables (or /2/2 cable) with independent neutrals for each hot beyond that point. You will not be able to put any 240V loads on that extension, sorry.