1. I do not believe the Monarch can act alone to modify Letters Patent that grant a title. I believe that legal precedent is if the Monarch attempts to do so, then it is treated as a fresh grant of the title. Where the Monarch has decided to change the Letters Patent, in "recent" years this has been via a complete regrant of the title - see e.g. The Duke of Fife, originally created in 1889 with remainder to heirs male, but later recreated with a special remainder to his daughters in 1900 when it became clear he would have no sons; from 1900 to 1912 there were two Dukedoms of Fife vested in the same person. Parliament is of course sovereign, so can choose to pass a law changing all letters patent, or vesting some power to modify letters patent in the Monarch
2. If Parliament passed such a provision, yes. However, this might be inadvisable as it would lead to the separation of senior titles from their historical subsidiary titles which were inherited on collateral lines as they pre-dated the senior title.
3. There has been some discussion of this in Parliament - the fact that there are many titles that already pass through female lines, and there is little desire to disturb this; the fact that some titles have even more exotic remainders, such as the Earldom of Selkirk, might not wish to be disturbed either.