You can try this - see below. Note that you have a duplicate variant position (12907457). I decided to merge these together first, as you can see.
awk 'BEGIN{FS=" "}; {a[$1]=a[$1]==""? $2 : a[$1] "," $2 } END {for (i in a) print i, a[i]}' lookupfile2.txt > lookupfile2.VarsMerged.txt
cat lookupfile2.VarsMerged.txt
12907379 CA
12907457 TC,GA
12837371 AA
11854476 GT
149275986 TT
1269554 CA
1372268 AT
1849529 GC
11854085 AG
awk 'BEGIN {FS=" "} FNR==NR {values[NR]=$0; next} header=0; /^pos/{start=$3; end=$4; header=1} {for (i in values) {split(values[i],a," "); if (header==1 && start<=a[1] && end>=a[1]) {print $0"\t"values[i]}; if (header==0 && start<=a[1] && end>=a[1]) {print $0}}}' lookupfile2.VarsMerged.txt bigfile1.txt
pos100 5 149275986 149275987 sample1 149275986 TT
1 NN
15 NN
18 NN
pos10 1 12907457 12907458 sample2 12907457 TC,GA
1 NN
2 NN
3 NN
18 NN
191 NN
pos100 5 149275986 149275987 sample3 149275986 TT
17 NN
15 NN
18 NN