No - the headlight system on the 964 does not use relays and the full headlamp current is passed through the dip switch on the steering column. Installing 100 watt bulbs will cause an increase in that current and will result in the failure of the switch and/or, potentially, a fire.

Yes, there are many dual beam H4 Xenon conversion kits available to provide both dipped and main beam improvement. There is limited room inside the headlamp bowl so you should ensure that you purchase a compact bulb set. Most successful installations place the ballasts behind the wheel arch protector although some place it inside the luggage compartment.

Also, be aware that in the UK you cannot legally fit Xenon lights unless you have self-levelling headlamps or suspension and headlamp washers. Although these are both legal requirements it seems that there have been few, if any, cases of MOT failure if not fitted.

You might also like to consider a recent investigation carried out by Auto Express (YouTube video here) which explains why such kits are illegal when fitted to light units designed for halogen bulbs.

The alignment adjusters. Make sure they are not confused with the mounting screw - just a single screw at the bottom of the bezel.

Probable loss of vacuum, check hoses on throttle linkage/behind airbox reservior and behind headlamp.