Questions about Jagen IVs/Build advice

I'm a bit confused as to why you say its a long investment project and getting him +10, but won't invest in a good weapon on him (like the berkut's lance mage tank build), or DC, which is what makes him somewhat viable when compared to other blue cavaliers that would probably be better in the long run...

Anyways, -Hp is probably the better option for him if you aren't fully investing the mage tank path. Slaying lance would probably be best of the options you gave, as it will help him get off specials more often which is essentially his only source of significant damage.

If you're not one of those peeps that are all about "ORKO only matters", please read on
-hp for sure. There are so many physical units out there, I don't get why you would want to have a -def Jagen against the next Ayra or Ike. Personally I prefer adding more defensive stats than raw hp because Jagen's stats should be pushed in betr places imo c;
I run a neutral def Felicia (summoner support +4 as we speak) with a defense refined rogue dagger (+3 Defense), double fortress defense (A and S slot) with great success. Sure she only has a measly 33 base defense anyways, but when she gets her rogue dagger up (+6 Defense) and starts sitting at 40 defense and then ally supported with a refined Corrin w/ Yato to 44 defense, hilarity ensues. Her attack is complete daisy's at 29, but I made my goal of turning her into a annoying defensive wall with both DEF and RES sitting above 40. I trash basic attacks altogether in favor of massive chip damage through specials instead: she tends to brawl things out.
I mention Felicia because Jagen has pretty darn similar defensive stats; high resistance, crap defense. Although who cares what's optimal and not, there's other viable builds Jagen can fulfill like my hyper defense Felicia does.
Seals and other def passives can really, REALLY fix Jagen's so-called poor defense (I mean look at my Felicia): a hyper def Jagen is totally viable and imo underrated. I imagine you might not run double fortress defense on your Jagen since you actually want to damage outside special damage, but a neutral def Jagen with both +3 defense slots is still 31 defense. Add a refined weapon, 35 defense, cavalry buffs, 41 defense, not to mention you're also going to have the same level of resistance (35 base res +6 cavalry buff = 41). Quick Riposte, Slaying Lance, Iceberg, and now you have a Jagen that might not be so afraid from taking a lucky hit from any green axe or any other green for that matter all the while happily taking hits from pretty much any red and happily face taking a hit from either Reinhardt or Bow Lyn

tl;dr I would recommend using the +ATK -HP Jagen and build him with mixed defenses. Distant counter/close counter overrated, try different approaches, don't be afraid to disregard the optimal stats entirely for something else