I.e. it means that only additional restrictions are added (in the case of segwit, that the witness must be valid in addition to the old-style signature). This means that old nodes accept all new blocks,
yep
(but before you said "old nodes will accept new rules", which is not the same, and self-evidently not possible)
but it also means that transactions that would be accepted by old nodes may be invalid according to the new rules.
nope, you're wrong
if nodes that observe a soft-fork's rules rejected blocks containing previously valid transactions, that would fork the chain. which isn't possible in this case, because the segwit soft-fork does not reject any rules that were valid before it's activation, that's the definition of a soft fork
but I don't see much point in arguing this further, tell us which blocks have been rejected by any soft-fork's rule set, that would constitute evidence of your claimaren't segwit blocks supposed to have a new commitment in the coinbase, which old mining software wouldn't create?
yes.
But the old way of doing it is still valid,
because soft-fork