Not sure I understand what point multiplication is (+s, -s and *s) but are you saying that the algorithm just applies a process of addition, subtraction and multiplication to the private key to obtain the public key?
Yes
Point multiplication is something specific to elliptic curves
Also on your second point, who (or what) determine the "target" for each block? I read somewhere that it's determined by the network difficulty but that doesn't make sense to me. Who (or what) sets the target for each block and who (or what) verifies the hashes until said target has been found?
The difficulty or target are similar, you can get one if you have the other
The difficulty is the same for sets of 2016 blocks
The first difficulty (blocks 1 to 2016) was 1
The recalculated difficulties (i.e. for blocks 2017, 4033, ..., n*2016+1) depends on the time between the 2016 previous blocks: it's a way to adjust the difficulty according to the numbers of miners
And If a computer can hash data easily, and it knows the network difficulty, can't it just work out which nonce to use rather than try heaps of combinations?
That's the point of mining: hashing is really fast, but the reverse operation is impossible