small batch ag questions

So here's the plan, been looking into making the transition to ag but don't want to buy new equipment until I move to a new apartment in four months~
I have a large rectangular cooler, a five gallon pot, and a weak stove that can barely boil 3.5 gallons.

2.5 gallon ag batches.

First question. Can I just clean the rect cooler and use a combination biab and igloo mlt setup by bagging the grain, and mashing in the cooler instead of the pot in order to control and maintain temps better? Then just siphon it out, or use the preexisting spigot?
If I can find a proper setup to hold the bag in place without spilling the grains it seems easier than installing a manifold/braid/false bottom and what not. Maybe a ss steam basket will work.

I don't really understand the purpose of a hlt. Why is it necessary, can't you just use your brewpot?

Your plan seems sound but something under the bag would be best for better drainage. Siphoning would be a PITA, the built in valve would work better, especially if you can lock it open somehow while draining. The HLT is really required when fly sparging since you need a hot liquor supply for sparging while the kettle is filling from the MLT.

Oh that makes more sense on the fly sparging.
I'd probably end up both siphoning and using the spigot. Do you mean under the bag while it's in the cooler in order to keep it off the bottom in order to improve extraction by exposing greatee surface area, or mean under in order to support the grains when draining and squeezing the bag?

you definitely need some space under the bag to get runoff. i have tried (quite) a few times to do 4L / 1G fusion BIAB / infusion mashes like what you describe in a small cooler using a small rack under the bag (as false bottom) and a siphon, but every time the whole thing was more trouble than it was worth; i lost the siphon (wort runoff is too slow to hold the siphon), the hose flew out, the wort didn't run, the rack collapsed (very poor design on that one), i lost mash temp due to opening a million times to fiddle with it, i gave up on it and just do regular BIAB, it's so easy if you can just hold temp in a pot on your weak stove, that's what i happily do now for small/test batches, but mine are much smaller than yours and i can runoff in a pasta strainer. batch sparge by dunking the bag in a second pot of sparge temp water.

i didn't have a spigot in my cooler, and never made a very good false bottom. if i were you i'd focus on those and forget the siphon.

Make this simple. Do your BIAB mash in the pot you have. Wrap the pot with something to insulate it and don't unwrap it until the mash is over. I use a bath towel and lose less than 2 degrees in an hour long mash. If you have some iodine available, do a starch conversion test with it at 30 minutes. That may be all the mash period you need if your grains are properly milled. Don't try to heat the pot until the grains are out. It will melt the bag to the bottom of the pot and unless you have a way to stir a lot you will get hot spots that will mess up your conversion.

You don't need an MLT. Just drain the wort into a bucket while using your pot for hot water. Once you've collected all the wort, just pour it into the pot and start boiling.

I'll double check the temp loss over an hour with a test run of hot water in the pot wrapped in some towels. If it's too much for my comfort, I'll do another run in a semi-full cooler.

Measure the displacement of the pot+"pre-wort" and fill the cooler until 155~ then put the pot into the cooler+water and use as a thermal heatsink to keep the temp stable.

One thing worth reading is the countertop partial mashing article from BYO (link below). The author seems to have had success using the spigot on a cooler. I have been doing standard BIAB partial mash for a few batches now but recently got a cheap 2gal beverage cooler to try this with. I have not done it yet though so cannot report any direct experience (hopefully in a couple weeks)

http://byo.com/stories/item/507-countertop-partial-mashing

I do 2 gallon batches on my stove all the time. I mash directly in the pot, just drape my 5 gallon bag in there and stir in once I hit temp. I then cover and and put my pot in the oven which I've preheated to its lowest setting, 170 in my case. Never lost a degree during mash, usually reach full conversion in about a half hour. I then pull the pot out, remove the bag and place a large colander over the pot. The bag sits in the colander and drains in about 10 - 15 minutes. Remove the bag, top off and boil. Really no need to sparge, but I guess you could if you wanted too.