I’m pretty sure on the Aruba stack you’ll find VLAN 99 configured with address 172.17.99.1. If not then you should find where 172.17.99.1 is, as that seems to be doing most of your routing.
As you said, you can create VLAN 99 (make sure the MX is in VLANs mode, not single VLAN) on your MX with the IP address of 172.17.99.3/24 - assuming nothing else is using it - which will make testing easy, followed by replacement of the Checkpoint. You then need the switch port on the MX that will connect to the Aruba stack configured the same way as the port on the Aruba stack that connects to the MX - I would tend to go with a trunk, native (or untagged) VLAN as 99, allow all VLANs.
Your static routes on the MX should have a gateway IP of 172.17.99.1, so that traffic that is intended for the remote VLANs is sent to the next hop - which should be the same next hop that the Checkpoint currently uses (probably the Aruba stack). Then as you say you can test by changing the Default Route on the Aruba stack to 172.17.99.3 (it’s probably currently pointing to the Checkpoint on 172.17.99.2). You can test by trying to Ping the MX (172.17.99.3) from your 10.1.1.0/24 Corporate network, if that works you’re heading the right way.