RobinBobinBarabek

Можно сделать с помощью прозрачного border'а:

function pairwise(array) {
  const result = [];

  for (let i = 0; i < array.length; i += 2) {
    result.push({ x: array[i], y: array[i + 1] });
  }

  return result;
}

const polydata = 'MULTIPOLYGON(((-61.2811499892424 -51.8650394644694,-61.3156095883879 -51.84003983505,-61.3451180293736 -51.836359409781,-61.345979 -51.8243193384298,-61.2230287013809 -51.8586600606699,-61.2380183615625 -51.8776,-61.2792595470359 -51.8659093831693,-61.2811499892424 -51.8650394644694)),((-59.6938009153582 -52.2130104476997,-59.6697 -52.2216295490501,-59.6888996395972 -52.2286288414282,-59.6929605766079 -52.2129007110094,-59.6938009153582 -52.2130104476997),(-59.7427025342573 -52.2157144418542,-59.7402989592584 -52.226169308217,-59.7414397191119 -52.2143487582055,-59.7427025342573 -52.2157144418542)),((-58.4328292075227 -52.102969,-58.4668596645304 -52.094089483711,-58.444380216144 -52.0871892532998,-58.4332186231022 -52.1024482278935,-58.4328292075227 -52.102969)))';

const polystr = polydata.replace(/MULTIPOLYGON|\(|\)|\s+|\.-/g, (ch) => {
  if (ch === 'MULTIPOLYGON') return '';
  else if (ch === '.-') return '.0,-';
  else if (ch === '(') return '[';
  else if (ch === ')') return ']';
  else if (ch[0] === ' ') return ',';
});

const polyarray = JSON.parse(polystr).map(arr => arr.map(a => pairwise(a)));

const { shapes, holes } = polyarray.reduce((acc, val) => {
  if (val.length === 1) {
    acc.shapes.push(...val[0]);
  }
  else if (val.length > 1) {
    acc.holes.push(...val[0]);
    acc.shapes.push(...val.slice(1).flat());
  }

  return acc;
}, { shapes: [], holes: [] });

console.log('shapes =>', shapes);
console.log('holes =>', holes);

let data = 'MULTIPOLYGON(((-61.2811499892424 -51.8650394644694, -61.2811499892424 -51.8650394644694)),((-59.6938009153582 -52.2130104476997, ( -59.6929605766079 -52.2129007110094,-59.6938009153582 -52.2130104476997),((-51.3119402806134,-59.5789385998322 -51.3125898252842)))';
data = data.replace(/\(*\)*/gm, '');
console.log(data);

Если правильно понял, в массиве лежит «кольцо» точек. Первая соседствует с последней.
И к любой выбранной точке надо добавить две новые, слева и справа, с усреднёнными с соседями координатами.

function addPointCenter(index) {
  const avg = (a, b) => (a + b) / 2;
  const midPoint = (p1, p2) => ({ lat: avg(p1.lat, p2.lat), lng: avg(p1.lng, p2.lng) });
  const insert = (index, point) => latlng.splice(index, 0, point);

  const { length } = latlng;
  const current = latlng[index];

  const indexLeft = (index - 1 + length) % length;
  const pointLeft = midPoint(current, latlng[indexLeft]);

  const indexRight = (index + 1) % length;
  const pointRight = midPoint(current, latlng[indexRight]);

  insert(index + 1, pointRight);
  insert(index, pointLeft);

  drawMarkers();
}