1. Can SPs of a Routed force retreat into a Fort which is in the original battle space?
Yes; the Battle of Prag in the Extended Example of Play illustrates this. The fifth bullet of 15.9 makes no distinction among reasons for the retreat.
2. If a besieging Command tries to make a Siege roll, it can be Intercepted and a Battle ensues. If this happens in the Strategy Card segment then presumably whatever the outcome the besiegers have now fought a Battle and so cannot make the Siege roll [17.4 bullet 1]. What happens if the Interception occurs during the Siege Segment?
If the intercepting Command is forced to retreat while the besiegers are not and still have sufficient SPs and artillery, the active Power may roll in the Siege Segment. Both 5.1.2b(2c) and 17.4 specifically relate to the Strategy Card Segment. 17.4.1 states, "During a Siege Segment, all Commands controlled by the active Power in spaces with Siege markers and no enemy force outside the fortress may make one Siege roll per fortress." [emphasis included]
Think of the roll during a Strategy Card Segment as active, vigorous prosecution of the siege, hence the requirement to activate the besieging Command and expend an SA. Having to interrupt this activity to fight a field battle is sufficient to negate the roll. The roll during the Siege Segment represents more the passage of time, with the resulting decrease in the garrison's ammunition, provisions, and manpower and erosion of their morale.