The MiG-29 factory at Sokol used parts from old parts stockpiles still in storage to make the MiG-29SMTs. When Algeria found that out they returned them to the factory. The MiG-29SMTs were then bought by the Russian Air Force. Last I heard they were deployed in an airbase the Russians have in Armenia.
As for why they bought the Su-30MKA as a replacement, back then Sukhoi and Mikoyan operated as separate companies. Sukhoi basically took advantage of the dissatisfaction of the Algerians with the MiG-29SMT deal and made them an offer to buy the Su-30MKA at a discounted price. Sukhoi basically shafted Mikoyan. Would not happen today when they are both part of UAC.
Yeah it was really crummy for Mikoyan. This was their biggest deal back then and it also probably impacted sales to other clients. They expected to make money on MiG-29SMT sales and upgrades of older MiG-29s but the bad publicity plus bargain basement Flanker sales shafted them. While Mikoyan did eventually get the MiG-31BM contract and sold some MiG-29Ms to Egypt it is just a pale shadow of what their sales used to be.
It is kind of unlikely. Belarus can upgrade and maintain the MiG-29 by themselves and they even do it for foreign clients. They will probably continue using those aircraft until they fail completely. Plus like you said they can get advanced Flanker variants. As for CAATSA Belarus does not care about it, it is also sanctioned to hell and back just like Russia. Plus half their trade volume both imports and exports is with Russia so it shouldn't be too hard to pay them back.
