How to add lines on combined ggplots, from points on one plot to points on the other?

My solution is a little ad hoc, but it seems to work. I based it on the following previous answer Left align two graph edges (ggplot).

I will break the solution in three parts to address some of the issues you were facing separately.

The solution that matches what you want is the third one!

First trial

Here I get the axis aligned using the same approach as this answer Left align two graph edges (ggplot).

# first trial 
# plots are aligned but line in bottom plot extends to the bottom
#
p1_1 <- p1 +
  annotation_custom(GROB,
                    xmin = 0,
                    xmax = POINTS$quantity[POINTS$label == "B"],
                    ymin = POINTS$value[POINTS$label == "B"],
                    ymax = POINTS$value[POINTS$label == "B"]) +
  annotation_custom(GROB,
                    xmin = POINTS$quantity[POINTS$label == "B"],
                    xmax = POINTS$quantity[POINTS$label == "B"],
                    ymin = 0,
                    ymax = POINTS$value[POINTS$label == "B"]) +
  geom_point(data = POINTS %>% filter(label == "B"), size = 1)

p2_1 <- p2 + annotation_custom(GROB,
                               xmin = POINTS$quantity[POINTS$label == "B"],
                               xmax = POINTS$quantity[POINTS$label == "B"],
                               ymin = POINTS$profit[POINTS$label == "B"],
                               ymax = GROB_MAX)

# Create gtable from ggplot
gA <- ggplotGrob(p1_1)
gB <- ggplotGrob(p2_1)

# Turn clip off for panel so that line can extend above
gB$layout$clip[gB$layout$name == "panel"] <- "off"

# get max width of left axis between both plots
maxWidth = grid::unit.pmax(gA$widths[2:5], gB$widths[2:5])

# set maxWidth to both plots (to align left axis)
gA$widths[2:5] <- as.list(maxWidth)
gB$widths[2:5] <- as.list(maxWidth)

# now apply all widths from plot A to plot B 
# (this is specific to your case because we know plot A is the one with the legend)
gB$widths <- gA$widths

grid.arrange(gA, gB, ncol=1)

Second trial

The problem now is that the line in the bottom plot extends beyond the plot area. One way to deal with this is to change coord_cartesian() to scale_y_continuous() and scale_x_continuous() because this will remove data that falls out of the plot area.

# second trial 
# using scale_y_continuous and scale_x_continuous to remove data out of plot limits
# (this could resolve the problem of the bottom plot, but creates another problem)
#
p1_2 <- p1_1 

p2_2 <- data_long %>%
  filter(variable == "profit") %>%
  ggplot(aes(x = quantity, y = value)) +
  geom_line(color = "darkgreen") +
  scale_x_continuous(limits = c(0, 20), expand = c(0, 0)) +
  scale_y_continuous(limits=c(-100, 120), expand=c(0,0)) +
  theme(legend.position = "none") + 
  annotation_custom(GROB,
                    xmin = POINTS$quantity[POINTS$label == "B"],
                    xmax = POINTS$quantity[POINTS$label == "B"],
                    ymin = POINTS$profit[POINTS$label == "B"],
                    ymax = GROB_MAX)

# Create gtable from ggplot
gA <- ggplotGrob(p1_2)
gB <- ggplotGrob(p2_2)

# Turn clip off for panel so that line can extend above
gB$layout$clip[gB$layout$name == "panel"] <- "off"


# get max width of left axis between both plots
maxWidth = grid::unit.pmax(gA$widths[2:5], gB$widths[2:5])

# set maxWidth to both plots (to align left axis)
gA$widths[2:5] <- as.list(maxWidth)
gB$widths[2:5] <- as.list(maxWidth)

# now apply all widths from plot A to plot B 
# (this is specific to your case because we know plot A is the one with the legend)
gB$widths <- gA$widths

# but now the line does not go all the way to the bottom y axis
grid.arrange(gA, gB, ncol=1)

Third trial

The problem now is that the line does not extend all the way to the bottom of the y-axis (because the point below y=-100 was removed). The way I solved this (very ad hoc) was to interpolate the point at y=-100 and add this to the data frame.

# third trial 
# modify the data set so value data stops at bottom of plot
# 
p1_3 <- p1_1 

# use approx() function to interpolate value of x when y value == -100
xvalue <- approx(x=data_long$value, y=data_long$quantity, xout=-100)$y

p2_3 <- data_long %>%
  filter(variable == "profit") %>%
  # add row with interpolated point!
  rbind(data.frame(quantity=xvalue, variable = "profit", value=-100)) %>%
  ggplot(aes(x = quantity, y = value)) +
  geom_line(color = "darkgreen") +
  scale_x_continuous(limits = c(0, 20), expand = c(0, 0)) +
  scale_y_continuous(limits=c(-100, 120), expand=c(0,0)) +
  theme(legend.position = "none") + 
  annotation_custom(GROB,
                    xmin = POINTS$quantity[POINTS$label == "B"],
                    xmax = POINTS$quantity[POINTS$label == "B"],
                    ymin = POINTS$profit[POINTS$label == "B"],
                    ymax = GROB_MAX)

# Create gtable from ggplot
gA <- ggplotGrob(p1_3)
gB <- ggplotGrob(p2_3)

# Turn clip off for panel so that line can extend above
gB$layout$clip[gB$layout$name == "panel"] <- "off"


# get max width of left axis between both plots
maxWidth = grid::unit.pmax(gA$widths[2:5], gB$widths[2:5])

# set maxWidth to both plots (to align left axis)
gA$widths[2:5] <- as.list(maxWidth)
gB$widths[2:5] <- as.list(maxWidth)

# now apply all widths from plot A to plot B 
# (this is specific to your case because we know plot A is the one with the legend)
gB$widths <- gA$widths

# Now line goes all the way to the bottom y axis
grid.arrange(gA, gB, ncol=1)

This makes use of facet_grid to force the x-axis to match.

grobbing_lines <- tribble(
  ~facet,   ~x, ~xend,       ~y,    ~yend,
  'profit',  5,     5,       50,      Inf,
  # 'curve',   5,     5,     -Inf, 28.39397
  'curve',   -Inf,     5, 28.39397, 28.39397
)

grobbing_points <- tribble(
  ~facet,   ~x,        ~y,    
  'curve',   5,  28.39397 
)

data_long_facet <- data_long%>%
  mutate(facet = if_else(variable == 'profit', 'profit', 'curve'))

p <- ggplot(data_long_facet, aes(x = quantity, y = value)) +
  geom_line(aes(color = variable))+
  facet_grid(rows = vars(facet), scales = 'free_y')+
  geom_segment(data = grobbing_lines, aes(x = x, xend = xend, y = y, yend = yend),inherit.aes = F)+
  geom_point(data = grobbing_points, aes(x = x, y = y), size = 3, inherit.aes = F)

pb <- ggplot_build(p)
pg <- ggplot_gtable(pb)

#formulas to determine points in x and y locations
data2npc <- function(x, panel = 1L, axis = "x") {
  range <- pb$layout$panel_params[[panel]][[paste0(axis,".range")]]
  scales::rescale(c(range, x), c(0,1))[-c(1,2)]
}

data_y_2npc <- function(y, panel, axis = 'y') {
  range <- pb$layout$panel_params[[panel]][[paste0(axis,".range")]]
  scales::rescale(c(range, y), c(0,1))[-c(1,2)]
}


# add the new grob
pg <- gtable_add_grob(pg,
                      segmentsGrob(x0 = data2npc(5),
                                   x1 = data2npc(5),
                                   y0=data_y_2npc(50, panel = 2)/2,
                                   y1 = data_y_2npc(28.39397, panel = 1L)+ 0.25) ,
                      t = 7, b = 9, l = 5)

#print to page
grid.newpage()
grid.draw(pg)

The legend and the scales are what do not match your intended output.