1) The distinction is a little arbitrary, of course all players seek to maximise bb/100. But it is true that rakeback is especially significant for a shortstacker. In particular, if dealt rakeback is used, this is very advantageous for a shortstacker: he is paid rake from side pots he can't ever enter.

2) I believe that a 'never fold' strategy (which I take to mean, never releasing a hand once the decision is made to play it preflop) is profitable at lower stack sizes like 10bb. Indeed, when playing 10bb from BTN,CO or earlier it is often preferable to open to 3.5x, then shove any flop. If you are called preflop, there will be 7-8 bb in the pot and you will have 6.5 or less behind. This helps trap more deepstacked players in the hand - this is a huge part of your edge, getting money in multi-way allins vs deepstacked opponents (see pokershortstack.com Shortstacker’s Manifesto), because often they will be forced to forfeit their equity later in the hand due to pressure from other deepstacks (pressure you as a shortstacker do not experience).

Note that this strategy does not only open value hands. The 10bb Strategy outlined on my site will shove over 40% of hands when its folded to the shortstacker in SB. This is the game-theory-optimal strategy in this situation. Interestingly, this often tilts the other players at the table, many think it shoves T7s in SB to gamble, and will then call its EP opens (which are with a far stronger range) very light. For reference here are the opening ranges



I designed this 10bb strategy with the aim of getting value from the earlier positions, and maximising the inherent shortstack edge by inducing multi-way allins (its very easy for it to get allin for 10bb with 3.5x PFR + 6.5x flop jam). I am considering revising it to tighten up even more from EP, and letting its aggressive (but correct) LP play give it the table image to get max value in these seats.

However at 20bb I think that some postflop play is certainly necessary. I am currently in the process of writing strategies for this stacksize, and I will keep you updated. I think that push-fold poker at 20bb will only work if extremely tight ranges are used (eg so it can just happily shove 2 pots on almost any flop, or whatever), and will only be effective in terms of getting value from loose callers pre. But 20bb is a great stacksize for squeezing and 3betting preflop - eg when CO opens, BTN calls, shortstack shoves 20bb from BB with A9, very tasty +EV here since they are showing up with hands like 22 or 75s that cannot call the shove (22 cannot call because our shoving range includes many stronger hands too)

3) I would say that with 3+ shortstackers at 6max, the game breaks down, although the deepstacks may still fail to adapt.

4) This is an interesting question, I will take it that SB is always folding, and that the effective stacks between us are 20bb. I think it is not a terrible strategy for the BB to shove or fold in this case. I will adapt my nash equilibrium finder to calculate the optimal strategies for this game

(SB may open to 2.25x or fold, BB may jam 20bb over this raise or fold).

It would give a concrete starting point for the strategy.

Certainly you are right that it is best to open small (eg minraise) when the eff stacks are 20bb and you are in late position. This makes the BB jam-or-fold response simplification less appropriate, thereby forcing him to call OOP some. If you open to 4x for instance then the BB can happily just jam or fold (which of course negates your positional advantage postflop).