First question is very easy to answer, the block initial design objective was to make it a replacement for a 3.8 stock block and utilize all internals as well as bolt-on parts. Few changes like the 6-bolt mains and provision for 14-bolt heads. If only 8 are used, no problem, as none of these go into the water jackets.
Since T/A Performance now owns the rights to the M&A heads [and their other Buick products too], the mating of block and head will be much stronger. Result, harder to ruin a head gasket. The block deck surface and the heads have been made thicker to help eliminate deformation, or heads lifting under heavy boost conditions.
This leads into the second question. The block design has been to handle in excess of 1000 HP. This will be tough to achieve with all stock internals. Seems like the present reliable limit for a stock block is ~700 HP. The structural limit of the block with its thin-wall heritage, deforms, cracks, breaks and other unwanted horrors come out. The stock crank does rather well considering how it is tortured. If the block does NOT hold the crank in place, it gives up easy.
How will a stock crank hold up in the T/A block? Probably better than in a stock block, but hard to guess at this point.
At the 700+ HP levels we would be seeing this as a competition motor. So lots of good parts will be inside. Having seen some of the junk cranks being used [or trying!], it will be important to make sure you have a good one.
One more thought, T/A has been over 3 years just making the commitment to the turbo V-6 scene. Lots of effort and $$$$ have been poured into the involvement already. It will not stop with just a block and heads. With the support of the Buick community, T/A will develop, procure and purchase many other performance and basic items for the engines and cars too.
The ability to work with major manufactures and purchase in viable quantities will keep our cars running [and performing] for a great many years. Certainly billet cranks have been explored to go with this motor as have cams, pistons, rings, bearings and lots of other essentials that are already drying up in the marketplace.
It will take a company with the resources, experience and credibility like T/A Perf to make this happen. Again, this can only happen with the continued support of the Buick owners for street driving, racing, daily drivers or???
