I very much doubt that the 'red spiders' are the cause of your maple's
problems. At this time of year during fine weather, common garden
spiders (and probably many others) hatch out in considerable colonies
and are often found at the tips of stems on shrubs and trees. They
remain together for a few days and then disperse. The red spider mite
that causes problems in plants is a tiny, yellowish mite, barely
visible to the naked eye that is rarely a problem out of doors in the
UK and even then only after a very long hot dry spell. Looking at the
necrosis on the bark, which is far more serious; this is almost
certainly damage caused by verticillium wilt disease.

In Japanese maples it infects the roots initially and then travels up
through the plant, blocking sap conducting tissues under the bark and
resulting in dead patches on young bark as in the photographs. Leaves
yellow or shrivel, but remain attached and shoot tips wilt as a result
of the sap flow being severely restricted or cut off in extreme
cases. Unfortunately there is no real cure, although plants can often
recover if they are encouraged to grow strongly and produce more
conductive tissue. Giving regular applications of a seaweed based
liquid fertiliser should help and make sure that your plant is well
watered during dry weather. If the stems become completely 'girdled',
then cut them back to clean wood, but it is possible for a branch to
survive and recover if there is only partial 'girdling'. Apart from
that all you can do is keep your fingers crossed.