You need gallow brackets and a concrete or steel lintil
If by restrain you mean support the stack(s) above, then absolutely not.
No you wouldn't be allowed to do that when taking any load bearing wall out you would need a structural engineer in first who will tell you what size RSJ / support to use then once the work is completed by your builder you would get building control to come and pass it / make sure it's been done properly.
No. The timber floor is flexable. It will be bouncing under the chimney breast and will cause some movement.